NullReferenceException when using "Destroy all helper meshes in scene" option

[loosegrid]

At line 625.

Cause:
Collecting all root gameobjects with MeshFilters, but not checking if they actually have a mesh assigned. So allMeshesInScene contains null values

Fix
Check for null.

Related:

• Destroys mesh but not gameobject. (should be marked DontSave but that's another known issue)
• Are helper meshes not saved somewhere when they're generated?
• This feature can probably double as a temporary fix for Redundant [default-CSGModel] is created on domain reload in wrong scene when using additive scene loading #386 and other DontSave objects being saved for some reason
• Should probably set the toolbar back to "Show Only Visible" when destroying