See #17465 (dist): IAE: Listener buildings_tools.DrawBuildingAction was not registered before or already removed.
This occurs when a user attempts to enter the mapmode when the edit dataset is
null. We fix the problem by (a) updating the layerIsSupported method to use
isEditableDataLayer (introduced in r13434) and (b) removing a check that used
to ensure that if a user entered the mapmode when there was no editable layer,
we immediately exit from the mapmode and enter into the select mode. As all
calls to enterMode should be guarded by layerIsSupported (on layer change),
this is safe now that we are checking if the layer is editable.
