Fix crash when toggling media

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

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

@@ -341,7 +341,7 @@ public class CallActivity extends AppCompatActivity {
                 }
             }
 
-            if (localMediaStream.videoTracks.size() > 0) {
+            if (localMediaStream != null && localMediaStream.videoTracks.size() > 0) {
                 localMediaStream.videoTracks.get(0).setEnabled(enable);
             }
 
@@ -356,7 +356,7 @@ public class CallActivity extends AppCompatActivity {
                 message = "audioOn";
             }
 
-            if (localMediaStream.audioTracks.size() > 0) {
+            if (localMediaStream != null && localMediaStream.audioTracks.size() > 0) {
                 localMediaStream.audioTracks.get(0).setEnabled(enable);
             }
         }