ResizeListener: Fix null reference

[Habetdin]

Description

Simple fix of the null reference described in #6128 as the Issue 5

How Has This Been Tested?

Visually

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist:

• The PR is submitted to the correct branch (dev).
• My code follows the code style of this project.
• I've added relevant tests.

[codecov]

Codecov Report

Base: 91.51% // Head: 91.50% // Decreases project coverage by -0.01% ⚠️

Coverage data is based on head (0c08ab5) compared to base (b9bc15d).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##              dev    #6302      +/-   ##
==========================================
- Coverage   91.51%   91.50%   -0.01%     
==========================================
  Files         384      384              
  Lines       15066    15067       +1     
==========================================
  Hits        13787    13787              
- Misses       1279     1280       +1     

Impacted Files	Coverage Δ
...MudBlazor/Services/ResizeListener/ResizeOptions.cs	91.30% <100.00%> (+0.39%)	⬆️
...r/Components/Autocomplete/MudAutocomplete.razor.cs	92.59% <0.00%> (-0.42%)	⬇️

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[Mr-Technician]

Closing favor of #6308.