fix: check mounted after async

[agent3bood]

Connection with issue(s)

Close #1210

Solution description

Check if the widget is still mounted after returning from async call.

Screenshots or Videos

To Do

• Read contributing guide
• Check the original issue to confirm it is fully satisfied
• Add solution description to help guide reviewers
• Add unit test to verify new or fixed behaviour
• If apply, add documentation to code properties and package readme

[codecov]

Codecov Report

Merging #1268 (fb9634e) into main (4498e6d) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #1268      +/-   ##
==========================================
+ Coverage   84.96%   84.98%   +0.02%     
==========================================
  Files          20       20              
  Lines         705      706       +1     
==========================================
+ Hits          599      600       +1     
  Misses        106      106              

Impacted Files	Coverage Δ
lib/src/fields/form_builder_date_time_picker.dart	86.27% <100.00%> (+0.13%)	⬆️

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