Add defaultHandler check for null

[mrblrrd]

Problem

 java.lang.NullPointerException: Attempt to invoke interface method 'void java.lang.Thread$UncaughtExceptionHandler.uncaughtException(java.lang.Thread, java.lang.Throwable)' on a null object reference
  at com.google.firebase.crashlytics.internal.common.CrashlyticsUncaughtExceptionHandler.uncaughtException(CrashlyticsUncaughtExceptionHandler.java:62)
  at com.example.CompositeUncaughtExceptionHandler.uncaughtException(CompositeUncaughtExceptionHandler.kt:37)
  at org.chromium.base.JavaExceptionReporter.uncaughtException(chromium-TrichromeWebViewGoogle6432.aab-stable-636718033:27)
  at java.lang.ThreadGroup.uncaughtException(ThreadGroup.java:1071)
  at java.lang.ThreadGroup.uncaughtException(ThreadGroup.java:1066)
  at java.lang.Thread.dispatchUncaughtException(Thread.java:2306)

Steps to reproduce

Apply bytecode transformation to substitute Thread.setDefaultUncaughtExceptionHandler() and Thread.getDefaultUncaughtExceptionHandler() calls with custom ones.

If you really want to 😅 If you don't please proceed to Relevant Code. The fix is trivial and safe.

@JvmStatic
fun setDefaultUncaughtExceptionHandler(exceptionHandler: Thread.UncaughtExceptionHandler) {
    if (compositeUncaughtExceptionHandlerIsAttached.compareAndSet(false, true)) {
        CompositeUncaughtExceptionHandler.systemUncaughtExceptionHandler = Thread.getDefaultUncaughtExceptionHandler()
        Thread.setDefaultUncaughtExceptionHandler(CompositeUncaughtExceptionHandler)
    }
    CompositeUncaughtExceptionHandler.addExceptionHandler(exceptionHandler)
}

@JvmStatic
fun getDefaultUncaughtExceptionHandler(): Thread.UncaughtExceptionHandler? {
    return null
}

The idea is to take control of app-internal UncaughtExceptionHandlers execution, add additional custom keys, etc.

So that the processing chain looks like

Relevant Code

I believe this code

    } finally {
      Logger.getLogger().d("Completed exception processing. Invoking default exception handler.");
      defaultHandler.uncaughtException(thread, ex);
      isHandlingException.set(false);
    }

should be

    } finally {
      Logger.getLogger().d("Completed exception processing. Invoking default exception handler.");
      if (defaultHandler != null) {
        defaultHandler.uncaughtException(thread, ex);
      }
      isHandlingException.set(false);
    }

to meet the Thread contract

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