Fix widgets not unsubscribing if type doesn't have a widget implementation

[Gold872]

If you drag a table containing a widget that doesn't have a widget implementation, it will never unsubscribe

This has the widget model be unsubscribed and disposed before returning null in the tree row's toWidgetContainerModel() method

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 3 lines in your changes are missing coverage. Please review.

Project coverage is 45.52%. Comparing base (b719a48) to head (03b2151).

Files	Patch %	Lines
...b/widgets/network_tree/networktables_tree_row.dart	0.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #43      +/-   ##
==========================================
- Coverage   45.54%   45.52%   -0.02%     
==========================================
  Files          73       73              
  Lines        7463     7466       +3     
==========================================
  Hits         3399     3399              
- Misses       4064     4067       +3     

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