Fix 5691: Add safety checks to each dialog call in the load-button handling

[Sleet01]

There was no code path handling cancellation or dealing with null objects.
Adds three such checks.

Testing:

• Tested with save file provided in issue
• Ran all 3 projects' unit tests

Close #5691

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 29.05%. Comparing base (36da675) to head (d038e37).
Report is 15 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #5815      +/-   ##
============================================
- Coverage     29.06%   29.05%   -0.01%     
+ Complexity    13911    13909       -2     
============================================
  Files          2507     2507              
  Lines        266343   266364      +21     
  Branches      47612    47620       +8     
============================================
- Hits          77400    77398       -2     
- Misses       185021   185029       +8     
- Partials       3922     3937      +15     

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