Guarding against nulls

Signed-off-by: Mario Danic <mario@lovelyhq.com>

app/src/main/java/com/nextcloud/talk/activities/CallActivity.java

@@ -993,8 +993,13 @@ public class CallActivity extends AppCompatActivity {
                 localMediaStream.dispose();
             }
 
-            localVideoTrack.dispose();
-            localAudioTrack.dispose();
+            if (localVideoTrack != null) {
+                localVideoTrack.dispose();
+            }
+
+            if (localAudioTrack != null) {
+                localAudioTrack.dispose();
+            }
 
             localVideoTrack = null;
             localAudioTrack = null;