[firebase_crashlytics] Fixes exception when trying to format crash re…

[josh-burton]

…ason when the crash has no context

Description

Seeing lots of crashes related to this issue.

Example crash log:

#0. Crashed: com.apple.main-thread
0  libsystem_platform.dylib       0x1ad46e384 _platform_strlen + 4
1  CoreFoundation                 0x1ad80b5e8 __CFStringAppendFormatCore + 6024
2  CoreFoundation                 0x1ad80d6b8 _CFStringCreateWithFormatAndArgumentsAux2 + 136
3  Foundation                     0x1ae1b6790 +[NSString stringWithFormat:] + 68
4  Runner                         0x100fa2608 -[FirebaseCrashlyticsPlugin handleMethodCall:result:] + 81 (FirebaseCrashlyticsPlugin.m:81)

Related Issues

Checklist

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• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• My PR includes unit or integration tests for all changed/updated/fixed behaviors (See Contributor Guide).
• All existing and new tests are passing.
• I updated/added relevant documentation (doc comments with ///).
• The analyzer (flutter analyze) does not report any problems on my PR.
• I read and followed the Flutter Style Guide.
• The title of the PR starts with the name of the plugin surrounded by square brackets, e.g. [shared_preferences]
• I updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy.
• I updated CHANGELOG.md to add a description of the change.
• I signed the CLA.
• I am willing to follow-up on review comments in a timely manner.

Breaking Change

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• Yes, this is a breaking change (please indicate a breaking change in CHANGELOG.md and increment major revision).
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[kroikie]

@athornz Thanks for the PR, could you update the description so it shows the error you are seeing when submitting reports?

[josh-burton]

@kroikie done!

[kroikie]

Once build passes I'll merge.