Nullpointer exception in CameraSource.java

[Huntler]

The release()-method in the CameraSource.java file causes a NullPointer exception, becuase the processingThread seems to be null. This should close Issue #71. I referred the direct link below

flutter_mobile_vision/android/src/main/java/io/github/edufolly/fluttermobilevision/ui/CameraSource.java

Line 1142 in 7019a96

assert (processingThread.getState() == State.TERMINATED);

[harrowmykel]

This has been fixed here

 https://github.com/harrowmykel/flutter_mobile_vision_2

He also migrated the plugin to non null.

https://pub.dev/packages/flutter_mobile_vision_2