Consistent crashing creating new file after using Pegbar Alignment under specific conditions

[Jose-Moreno]

Issue Summary

Pencil2D crashes once you use the pegbar alignment feature within a file that has at least 2 new layers of the same type (bitmap). For this crash to be reproducible you must create a new bitmap layer by hand or use another feature that automatically creates a new layer (such as the Image Predefined Set, see extra video below)

I believe this could also be tangentially related to the layer naming mismatch problem as reported here: #1414

Expected Results

Pencil2D should not crash when using the pegbar alignment feature nor after creating a new file for any reason.

Video or Image Reference

Minimum reproduction steps.
https://youtu.be/PjbLtdR-C2I

Pencil2D Crashing after importing with Image Predefined Set and using the Pegbar alignment feature.
https://youtu.be/V7ctw3lN0tQ

Steps to reproduce

1. Create new file
2. Create a new bitmap layer
3. Go to Edit > Pegbar Alignment > a dialog will open
4. Close the dialog immediately
5. Go to File > New
6. Silent crash

System Information

• Pencil2D Version:
  Version: 0.6.5

commit: 50a0c7c
date: 2020-08-01_01:32:15

Operating System: Windows 10 (10.0)
CPU Architecture: x86_64
Qt Version: 5.12.9

[davidlamhauge]

I looked into this yesterday, and there is a fairly simple solution, so I've assigned my self.
The problem is to close the dialog properly.