fix: Let's try to fix a crash with a null variable in the CameraScannerPageState

[g123k]

Hi everyone,

Following #4696 and with my tests during the POC, it seems that in release mode and the first time the screen is displayed, the size of the screen is set to zero. We simply have to wait a few frames.

I'm not sure if it will fix #4696, but now the algorithm retries to fetch the value + add a try/catch.

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[codecov-commenter]

Codecov Report

Merging #4713 (1c18faa) into develop (34f3364) will decrease coverage by 0.01%.
Report is 1 commits behind head on develop.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           develop   #4713      +/-   ##
==========================================
- Coverage     9.90%   9.90%   -0.01%     
==========================================
  Files          310     310              
  Lines        15807   15813       +6     
==========================================
  Hits          1566    1566              
- Misses       14241   14247       +6     

Files	Coverage Δ
...es/smooth_app/lib/pages/scan/camera_scan_page.dart	1.88% <0.00%> (-0.25%)	⬇️

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[monsieurtanuki]

Hi @g123k!
I would have removed altogether the concept of _headerHeight, but I'm notoriously not as pixel-precise as you are.
Not sure either that the PR will solve the initial issue (it looked like a matter of null rather than 0), but your try/catch will probably be helpful.