Thanks for the workaround, it worked [;)]
From your description, it sounds like an issue about the selected level of a home which is [font=courier new]null[/font] as long as the home doesn't contain any level.
At line 137 of [font=courier new]AutoDimensionigPlugin.java[/font], you create a new level in that case but then you don't update [font=courier new]selectedLevel[/font] field which makes [font=courier new]getWallsAtLevel[/font] fail line 143 when it tries to update [font=courier new]wallsAtLevel[/font] list.

Did you try to use Eclipse debugger (explained in the Debugging the plug-in section of the Plug-in developer's guide)?