In this usage of the AoE Trait, the Radius Marker’ field should contain the name of a Property (e.g. a Dynamic Property) that contains the radius of the AoE.
You have ‘25’ in that field, so Vassal is looking for a property named ‘25’ to find the radius in. This doesn’t exist, so it returns nothing, which is not an integer, hence the message. Vassal does not repeat duplicate error messages, so this is actually generating the error every time, you just don’t see the subsequent ones.
If the radius of the AoE is actually fixed at 25 hexes, then just click on ‘Fixed radius’ check box and you should be right.