Fix graph view adding N-D entities traceback

[PiispaH]

Fixed a bug that made the Connect entities -action in graph view not work.

Re #2772

Checklist before merging

• Documentation is up-to-date
• Release notes have been updated
• Unit tests have been added/updated accordingly
• Code has been formatted by black
• Unit tests pass

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 4 lines in your changes are missing coverage. Please review.

Project coverage is 65.76%. Comparing base (2e3ca53) to head (fac8711).

Files	Patch %	Lines
spinetoolbox/spine_db_icon_manager.py	0.00%	2 Missing and 1 partial ⚠️
spinetoolbox/spine_db_editor/graphics_items.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2777   +/-   ##
=======================================
  Coverage   65.76%   65.76%           
=======================================
  Files         196      196           
  Lines       35995    35998    +3     
  Branches     6118     6119    +1     
=======================================
+ Hits        23671    23673    +2     
- Misses      11055    11056    +1     
  Partials     1269     1269           

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