fix(modal): clear modal container ref on unmount

[myabeaver]

Description

This clears the modal container ref introduced here: #495

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules
• I have checked my code and corrected any misspellings

[codecov]

Codecov Report

Base: 97.45% // Head: 97.46% // Increases project coverage by +0.00% 🎉

Coverage data is based on head (73623c2) compared to base (a71573c).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #514   +/-   ##
=======================================
  Coverage   97.45%   97.46%           
=======================================
  Files         127      127           
  Lines        5505     5518   +13     
  Branches      441      443    +2     
=======================================
+ Hits         5365     5378   +13     
  Misses        140      140           

Impacted Files	Coverage Δ
src/lib/components/Modal/Modal.tsx	100.00% <100.00%> (ø)

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[myabeaver]

Changed lines of code for codecov to not report.