Hi,
it seems v.3.4.0 does not allow me to edit/add new extensions. I tried different versions of Java (Oracle 11 and 14 and openjdk 11) obtaining the same error. See below.
2020-09-07 11:49:01,175 [0-main] INFO VASSAL.launch.StartUp - Starting
2020-09-07 11:49:01,182 [0-main] INFO VASSAL.launch.StartUp - OS Linux 5.4.0-45-generic
2020-09-07 11:49:01,183 [0-main] INFO VASSAL.launch.StartUp - Java version 11.0.8
2020-09-07 11:49:01,184 [0-main] INFO VASSAL.launch.StartUp - VASSAL version 3.4.0
2020-09-07 11:49:01,417 [0-AWT-EventQueue-0] INFO VASSAL.launch.ModuleManager - Manager
2020-09-07 11:49:06,941 [0-SwingWorker-pool-1-thread-1] INFO VASSAL.launch.AbstractLaunchAction - Loading module file /home/giulio/Desktop/Varia/VASSAL modules/BoB-30M.vmod
2020-09-07 11:49:07,016 [0-SwingWorker-pool-1-thread-1] INFO VASSAL.launch.TilingHandler - No images to tile.
2020-09-07 11:49:07,016 [0-SwingWorker-pool-1-thread-1] INFO VASSAL.launch.AbstractLaunchAction - Loading module Band of Brothers
2020-09-07 11:49:07,019 [0-SwingWorker-pool-1-thread-1] INFO VASSAL.tools.io.ProcessLauncher - launching /usr/lib/jvm/java-11-oracle/bin/java -Xms256M -Xmx512M -DVASSAL.id=1 -DVASSAL.port=44295 -Duser.home=/home/giulio -Duser.dir=/home/giulio/Desktop/Varia/VASSAL-3.4.0 -cp /home/giulio/Desktop/Varia/VASSAL-3.4.0/lib/Vengine.jar VASSAL.launch.Editor --edit-extension -- /home/giulio/Desktop/Varia/VASSAL modules/BoB-30M.vmod /home/giulio/Desktop/Varia/VASSAL modules/BoB-30M_ext/inactive/ScreamingEagles
2020-09-07 11:49:07,411 [1-main] INFO VASSAL.launch.StartUp - Starting
2020-09-07 11:49:07,417 [1-main] INFO VASSAL.launch.StartUp - OS Linux 5.4.0-45-generic
2020-09-07 11:49:07,419 [1-main] INFO VASSAL.launch.StartUp - Java version 11.0.8
2020-09-07 11:49:07,419 [1-main] INFO VASSAL.launch.StartUp - VASSAL version 3.4.0
2020-09-07 11:49:07,419 [1-main] INFO VASSAL.launch.Launcher - Editor
2020-09-07 11:49:16,563 [1-AWT-EventQueue-0] WARN VASSAL.launch.BasicModule - Band of Brothers version 3.0Mbeta
2020-09-07 11:49:16,659 [1-AWT-EventQueue-0] ERROR VASSAL.tools.ErrorDialog -
java.lang.NullPointerException: null
at VASSAL.configure.ExtensionTree.isEditable(ExtensionTree.java:61)
at VASSAL.configure.ExtensionTree.isEditable(ExtensionTree.java:199)
at VASSAL.configure.ExtensionTree.updateEditMenu(ExtensionTree.java:392)
at VASSAL.configure.ConfigureTree.valueChanged(ConfigureTree.java:1142)
at java.desktop/javax.swing.JTree.fireValueChanged(JTree.java:2967)
at java.desktop/javax.swing.JTree$TreeSelectionRedirector.valueChanged(JTree.java:3456)
at java.desktop/javax.swing.tree.DefaultTreeSelectionModel.fireValueChanged(DefaultTreeSelectionModel.java:641)
at java.desktop/javax.swing.tree.DefaultTreeSelectionModel.notifyPathChange(DefaultTreeSelectionModel.java:1111)
at java.desktop/javax.swing.tree.DefaultTreeSelectionModel.setSelectionPaths(DefaultTreeSelectionModel.java:297)
at java.desktop/javax.swing.tree.DefaultTreeSelectionModel.setSelectionPath(DefaultTreeSelectionModel.java:191)
at java.desktop/javax.swing.JTree.setSelectionPath(JTree.java:1656)
at VASSAL.configure.ConfigureTree.<init>(ConfigureTree.java:206)
at VASSAL.configure.ExtensionTree.<init>(ExtensionTree.java:53)
at VASSAL.launch.ExtensionEditorWindow.<init>(ExtensionEditorWindow.java:39)
at VASSAL.launch.EditExtensionAction.loadModule(EditExtensionAction.java:59)
at VASSAL.launch.LoadModuleAction.performAction(LoadModuleAction.java:77)
at VASSAL.launch.Editor.launch(Editor.java:90)
at VASSAL.launch.Launcher$1.run(Launcher.java:150)
at java.desktop/java.awt.event.InvocationEvent.dispatch(InvocationEvent.java:313)
at java.desktop/java.awt.EventQueue.dispatchEventImpl(EventQueue.java:770)
at java.desktop/java.awt.EventQueue$4.run(EventQueue.java:721)
at java.desktop/java.awt.EventQueue$4.run(EventQueue.java:715)
at java.base/java.security.AccessController.doPrivileged(Native Method)
at java.base/java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:85)
at java.desktop/java.awt.EventQueue.dispatchEvent(EventQueue.java:740)
at java.desktop/java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:203)
at java.desktop/java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:124)
at java.desktop/java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:113)
at java.desktop/java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:109)
at java.desktop/java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:101)
at java.desktop/java.awt.EventDispatchThread.run(EventDispatchThread.java:90)
2020-09-07 11:49:35,937 [1-AWT-EventQueue-0] INFO VASSAL.build.GameModule - Exiting
2020-09-07 11:52:13,264 [0-SwingWorker-pool-1-thread-2] INFO VASSAL.launch.AbstractLaunchAction - Loading module file /home/giulio/Desktop/Varia/VASSAL modules/CCMedievalV1_5.vmod
2020-09-07 11:52:13,303 [0-SwingWorker-pool-1-thread-2] INFO VASSAL.launch.TilingHandler - No images to tile.
2020-09-07 11:52:13,304 [0-SwingWorker-pool-1-thread-2] INFO VASSAL.launch.AbstractLaunchAction - Loading module Commands & Colors Medieval
2020-09-07 11:52:13,305 [0-SwingWorker-pool-1-thread-2] INFO VASSAL.tools.io.ProcessLauncher - launching /usr/lib/jvm/java-11-oracle/bin/java -Xms256M -Xmx512M -DVASSAL.id=3 -DVASSAL.port=38271 -Duser.home=/home/giulio -Duser.dir=/home/giulio/Desktop/Varia/VASSAL-3.4.0 -cp /home/giulio/Desktop/Varia/VASSAL-3.4.0/lib/Vengine.jar VASSAL.launch.Editor --edit-extension -- /home/giulio/Desktop/Varia/VASSAL modules/CCMedievalV1_5.vmod /home/giulio/Desktop/Varia/VASSAL modules/CCMedievalV1_5_ext/inactive/flat_units_v0_1
2020-09-07 11:52:13,643 [3-main] INFO VASSAL.launch.StartUp - Starting
2020-09-07 11:52:13,649 [3-main] INFO VASSAL.launch.StartUp - OS Linux 5.4.0-45-generic
2020-09-07 11:52:13,651 [3-main] INFO VASSAL.launch.StartUp - Java version 11.0.8
2020-09-07 11:52:13,651 [3-main] INFO VASSAL.launch.StartUp - VASSAL version 3.4.0
2020-09-07 11:52:13,651 [3-main] INFO VASSAL.launch.Launcher - Editor
2020-09-07 11:52:23,208 [3-AWT-EventQueue-0] WARN VASSAL.launch.BasicModule - Commands & Colors Medieval version 1.5
2020-09-07 11:52:23,310 [3-AWT-EventQueue-0] ERROR VASSAL.tools.ErrorDialog -
java.lang.NullPointerException: null
at VASSAL.configure.ExtensionTree.isEditable(ExtensionTree.java:61)
at VASSAL.configure.ExtensionTree.isEditable(ExtensionTree.java:199)
at VASSAL.configure.ExtensionTree.updateEditMenu(ExtensionTree.java:392)
at VASSAL.configure.ConfigureTree.valueChanged(ConfigureTree.java:1142)
at java.desktop/javax.swing.JTree.fireValueChanged(JTree.java:2967)
at java.desktop/javax.swing.JTree$TreeSelectionRedirector.valueChanged(JTree.java:3456)
at java.desktop/javax.swing.tree.DefaultTreeSelectionModel.fireValueChanged(DefaultTreeSelectionModel.java:641)
at java.desktop/javax.swing.tree.DefaultTreeSelectionModel.notifyPathChange(DefaultTreeSelectionModel.java:1111)
at java.desktop/javax.swing.tree.DefaultTreeSelectionModel.setSelectionPaths(DefaultTreeSelectionModel.java:297)
at java.desktop/javax.swing.tree.DefaultTreeSelectionModel.setSelectionPath(DefaultTreeSelectionModel.java:191)
at java.desktop/javax.swing.JTree.setSelectionPath(JTree.java:1656)
at VASSAL.configure.ConfigureTree.<init>(ConfigureTree.java:206)
at VASSAL.configure.ExtensionTree.<init>(ExtensionTree.java:53)
at VASSAL.launch.ExtensionEditorWindow.<init>(ExtensionEditorWindow.java:39)
at VASSAL.launch.EditExtensionAction.loadModule(EditExtensionAction.java:59)
at VASSAL.launch.LoadModuleAction.performAction(LoadModuleAction.java:77)
at VASSAL.launch.Editor.launch(Editor.java:90)
at VASSAL.launch.Launcher$1.run(Launcher.java:150)
at java.desktop/java.awt.event.InvocationEvent.dispatch(InvocationEvent.java:313)
at java.desktop/java.awt.EventQueue.dispatchEventImpl(EventQueue.java:770)
at java.desktop/java.awt.EventQueue$4.run(EventQueue.java:721)
at java.desktop/java.awt.EventQueue$4.run(EventQueue.java:715)
at java.base/java.security.AccessController.doPrivileged(Native Method)
at java.base/java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:85)
at java.desktop/java.awt.EventQueue.dispatchEvent(EventQueue.java:740)
at java.desktop/java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:203)
at java.desktop/java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:124)
at java.desktop/java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:113)
at java.desktop/java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:109)
at java.desktop/java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:101)
at java.desktop/java.awt.EventDispatchThread.run(EventDispatchThread.java:90)