NullPointerException in case of error on opening native camera fixed.

modules/java/generator/src/java/android+NativeCameraView.java

@@ -53,6 +53,7 @@ public class NativeCameraView extends CameraBridgeViewBase {
         /* 1. We need to stop thread which updating the frames
          * 2. Stop camera and release it
          */
+        if (mThread != null) {
             try {
                 mStopThread = true;
                 mThread.join();
@@ -62,6 +63,7 @@ public class NativeCameraView extends CameraBridgeViewBase {
                 mThread =  null;
                 mStopThread = false;
             }
+        }
 
         /* Now release camera */
         releaseCamera();