[Chromium] Add null checks for MediaSessionDelegate access

The first thing the session does when closing a session is cleaning
up the session listeners. This involves, among other things, setting
all the delegates to null.

It's still possible to get media events from the web engine after we
proceed to close the session (like inactive notifications) so we must
ensure that we don't run any job in that case and ignore the notification.

We went the extra mile and we've also added null checks for a few
more insecure usages of getMediaSessionDelegate().

Fixes #915

app/src/common/chromium/com/igalia/wolvic/browser/api/impl/TabMediaSessionObserver.java

@@ -62,24 +62,30 @@ public void mediaSessionDestroyed() {
     @Override
     public void mediaSessionStateChanged(boolean isControllable, boolean isSuspended, boolean isActive) {
         assert mMediaSession != null;
+
+        // If the Session is being closed just ignore the change notification.
+        WMediaSession.Delegate delegate = mSession.getMediaSessionDelegate();
+        if (delegate == null)
+            return;
+
         if (isActive != mIsActive) {
             mIsActive = isActive;
             if (mIsActive) {
-                mSession.getMediaSessionDelegate().onActivated(mSession, mMediaSession);
+                delegate.onActivated(mSession, mMediaSession);
             } else {
                 stopUpdatingPosition();
-                mSession.getMediaSessionDelegate().onStop(mSession, mMediaSession);
-                mSession.getMediaSessionDelegate().onDeactivated(mSession, mMediaSession);
+                delegate.onStop(mSession, mMediaSession);
+                delegate.onDeactivated(mSession, mMediaSession);
             }
         }
         if (isSuspended != mIsSuspended) {
             mIsSuspended = isSuspended;
             if (!mIsSuspended) {
                 startUpdatingPosition();
-                mSession.getMediaSessionDelegate().onPlay(mSession, mMediaSession);
+                delegate.onPlay(mSession, mMediaSession);
             } else {
                 stopUpdatingPosition();
-                mSession.getMediaSessionDelegate().onPause(mSession, mMediaSession);
+                delegate.onPause(mSession, mMediaSession);
             }
         }
     }
@@ -116,8 +122,9 @@ public void mediaSessionPositionChanged(@Nullable MediaPosition position) {
 
     public void onMediaFullscreen(boolean isFullscreen) {
         assert mMediaSession != null;
-        mSession.getMediaSessionDelegate().onFullscreen(
-                mSession, mMediaSession, isFullscreen, null);
+        if (mSession.getMediaSessionDelegate() != null)
+            mSession.getMediaSessionDelegate().onFullscreen(
+                    mSession, mMediaSession, isFullscreen, null);
     }
 
     /* package */ class WMediaSessionImpl implements WMediaSession {
@@ -205,7 +212,7 @@ public void muteAudio(boolean mute) {
     }
 
     private void updatePosition() {
-        if (mMediaPosition == null || !mIsActive)
+        if (mMediaPosition == null || !mIsActive || mSession.getMediaSessionDelegate() == null)
             return;
 
         assert mMediaSession != null;
@@ -267,6 +274,8 @@ private Bitmap scaleBitmap(Bitmap image, int size) {
 
     private void updateMetaData() {
         assert mMediaSession != null;
+        if (mSession.getMediaSessionDelegate() == null)
+            return;
 
         WMediaSession.Metadata metadata;
         if (mMetadata != null) {