hotfix: concurrent modification in bus stop provider

[limwa]

This PR fixes a ConcurrentModificationError.

Stacktrace:

ConcurrentModificationError: Concurrent modification during iteration: _Map len:0.
  File "compact_hash.dart", line 714, in _CompactIterator.moveNext
  File "bus_stop_provider.dart", line 30, in BusStopProvider.fetchUserBusTrips
  File "<asynchronous suspension>"
  File "bus_stop_provider.dart", line 62, in BusStopProvider.removeUserBusStop

Review checklist

• Terms and conditions reflect the current change
• Contains enough appropriate tests
• If aimed at production, writes a new summary in whatsnew/whatsnew-pt-PT
• Properly adds an entry in changelog.md with the change
• If PR includes UI updates/additions, its description has screenshots
• Behavior is as expected
• Clean, well-structured code

[codecov]

Codecov Report

Merging #1197 (be1c3ae) into master (d12d444) will decrease coverage by 0%.
Report is 2 commits behind head on master.
The diff coverage is 0%.

Additional details and impacted files

@@          Coverage Diff           @@
##           master   #1197   +/-   ##
======================================
- Coverage      17%     17%   -0%     
======================================
  Files         229     229           
  Lines        6974    6974           
======================================
- Hits         1151    1144    -7     
- Misses       5823    5830    +7