Thanks for the workaround, it worked
From your description, it sounds like an issue about the selected level of a home which is null as long as the home doesn't contain any level.
At line 137 of AutoDimensionigPlugin.java, you create a new level in that case but then you don't update selectedLevel field which makes getWallsAtLevel fail line 143 when it tries to update wallsAtLevel list.
Did you try to use Eclipse debugger (explained in the Debugging the plug-in section of the Plug-in developer's guide)?
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Emmanuel Puybaret, Sweet Home 3D creator
