Fix error when background destroy

[cptbtptpbcptdtptp]

• this.texture = null; needs to be in front of this._material = null;
• add unit test

[codecov]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (b499f94) 65.10% compared to head (a185ff7) 65.11%.

❗ Current head a185ff7 differs from pull request most recent head 26423ae. Consider uploading reports for the commit 26423ae to get more accurate results

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1781   +/-   ##
=======================================
  Coverage   65.10%   65.11%           
=======================================
  Files         473      473           
  Lines       23641    23641           
  Branches     3362     3362           
=======================================
+ Hits        15392    15394    +2     
- Misses       7045     7047    +2     
+ Partials     1204     1200    -4     

Files Changed	Coverage Δ
packages/core/src/Background.ts	59.21% <100.00%> (+6.57%)	⬆️

... and 5 files with indirect coverage changes

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