Model corruption and crash after removing definition under certain conditions

[Eneroth3]

This one has been hunting my code for some time now and has been really hard to track down.

Steps to reproduce:

1. Open model containing a component, e.g. Architectural Template with Stacy.

2. Run this code:

   model = Sketchup.active_model
   definition = model.definitions.first
   model.start_operation("Reload", true)
   definition.entities.add_cpoint(ORIGIN)
   definition.parent.remove(definition)
   model.commit_operation
   Sketchup.undo # Can be replaced by unduoing from UI.

3. Trigger bugsplat by any of the following:
   a. Load component from component browser
   b. Create new component in model
   c. Run Fix Problems (Model Info > Statistics > Fix Problems)
   d. Save model

The crash can be prevented by doing any of the following:

1. Not use a wrapping operation
2. Not create the CPoint
3. Not call undo (or by redo the undo)

This bug was found when trying to make a workaround for #50.

New on SU2018 (DefinitionList.remove was added in 2018). Tested on Win 7.
Bugsplats have been reported by Eneroth3 on 2018-03-26.

[thomthom]

SU-40208

[DanRathbun]

A warning should be added to the docs for the DefinitionList#remove() method telling consumers not to do anything else (like adding new entities) to the definition within an operation where the #remove method is called.

Also, (in the warning) it should be strongly discouraged (because of this bug) for operations using the #remove method to be transparently attached to preceding operations.