A few examples
Basic Pieces which are entirely comprised of layers, so basicpiece trait is empty
Layers with multiple levels that follow properties where certain values require that the layer be blank/not visible - certain levels may be empty
Its easily solved by adding a transparent marker in each of them but doesn’t mean everyone is going to do that if they are lazy like me 
Should add it is only in the editor this occurs, but doesn’t affect module at run time so its really more an annoyance to designer which they can choose to ignore by checking the error box for now, no player is going to notice this
Example
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