fix: two small fixes to crash causing bugs

[SrGesus]

Description

• Play Pause plugin was missing a ImGui::End() call when ImGui::Begin() returns false.

• ECS would crash when an onRemove observer made a change to the entity

Checklist

• Self-review changes.
• Evaluate impact on the documentation.
• Ensure test coverage.
• Write new samples.
• Add entry to the changelog's unreleased section.

[codecov]

Codecov Report

Attention: Patch coverage is 50.00000% with 2 lines in your changes missing coverage. Please review.

Project coverage is 40.20%. Comparing base (4f2187d) to head (10f8d71).

Files with missing lines	Patch %	Lines
core/src/ecs/world.cpp	66.66%	1 Missing ⚠️
engine/src/tools/play_pause/plugin.cpp	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1347   +/-   ##
=======================================
  Coverage   40.19%   40.20%           
=======================================
  Files         437      437           
  Lines       31003    31006    +3     
=======================================
+ Hits        12463    12465    +2     
- Misses      18540    18541    +1     

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[SrGesus]

noticed identical problem in World::destroy