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In the case where `CreateImpellerContext` encounters an error while creating an `impeller::ContextMTL`, we logged an error, returned nullptr, then immediately dereferenced the null pointer. Now, rather than crash due to a segfault, we now intentionally abort with an appropriate error message. This adds checks in the `FlutterDarwinContextMetalImpeller` initialiser that aborts with an appropriate error message if impeller context creation fails, Metal device creation fails, or texture cache creation fails. Rather than bailing out and returning nil from the initialiser to pass the buck to the caller, we terminate since without a graphics context, the app won't be able to render anything to begin with. Issue: https://github.com/flutter/flutter/issues/157489 Issue: [b/378790930](http://b/378790930) No test changes since this just changes an accidental crash to an intentional crash. [C++, Objective-C, Java style guides]: https://github.com/flutter/engine/blob/main/CONTRIBUTING.md#style