Fix NPE in ThumbnailsTask.

The async task was being cancelled, but there was still a race condition
in how the thumbnails list was being managed. This attempts to fix that.

app/src/main/java/org/thoughtcrime/securesms/video/videoconverter/VideoThumbnailsView.java

@@ -21,6 +21,7 @@ import java.io.IOException;
 import java.lang.ref.WeakReference;
 import java.util.ArrayList;
 import java.util.Arrays;
+import java.util.List;
 
 @RequiresApi(api = 23)
 public class VideoThumbnailsView extends View {
@@ -28,7 +29,7 @@ public class VideoThumbnailsView extends View {
   private static final String TAG = Log.tag(VideoThumbnailsView.class);
 
   private          MediaInput                    input;
-  private ArrayList<Bitmap>             thumbnails;
+  private volatile ArrayList<Bitmap>             thumbnails;
   private          AsyncTask<Void, Bitmap, Void> thumbnailsTask;
   private          OnDurationListener            durationListener;
 
@@ -169,6 +170,7 @@ public class VideoThumbnailsView extends View {
     final float                              thumbnailWidth;
     final float                              thumbnailHeight;
     final int                                thumbnailCount;
+
     long duration;
 
     ThumbnailsTask(final @NonNull VideoThumbnailsView view, final @NonNull MediaInput input, final float thumbnailWidth, final float thumbnailHeight, final int thumbnailCount) {
@@ -191,8 +193,11 @@ public class VideoThumbnailsView extends View {
 
         @Override
         public boolean publishProgress(int index, Bitmap thumbnail) {
+          boolean notCanceled = !isCancelled();
+          if (notCanceled) {
             ThumbnailsTask.this.publishProgress(thumbnail);
-          return !isCancelled();
+          }
+          return notCanceled;
         }
 
         @Override
@@ -205,20 +210,26 @@ public class VideoThumbnailsView extends View {
 
     @Override
     protected void onProgressUpdate(Bitmap... values) {
-      final VideoThumbnailsView view = viewReference.get();
-      if (view != null) {
-        view.thumbnails.addAll(Arrays.asList(values));
+      if (isCancelled()) {
+        return;
+      }
+
+      VideoThumbnailsView view       = viewReference.get();
+      List<Bitmap>        thumbnails = view != null ? view.thumbnails : null;
+      if (thumbnails != null) {
+        thumbnails.addAll(Arrays.asList(values));
         view.invalidate();
       }
     }
 
     @Override
     protected void onPostExecute(Void result) {
-      final VideoThumbnailsView view = viewReference.get();
+      VideoThumbnailsView view       = viewReference.get();
+      List<Bitmap>        thumbnails = view != null ? view.thumbnails : null;
       if (view != null) {
         view.setDuration(duration);
         view.invalidate();
-        Log.i(TAG, "onPostExecute, we have " + view.thumbnails.size() + " thumbs");
+        Log.i(TAG, "onPostExecute, we have " + (thumbnails != null ? thumbnails.size() : "null") + " thumbs");
       }
     }
   }