Fix crash when setting participant layout before receiving the data

When setting the participant layout, if the HPB is not used, the user ID
is got from the participant list data fetched when a new participant is
found. However, as the participant layout is setup as soon as a new
participant is found the data may have not been received yet, which
ended in a crash (NullPointerException). Now the access to the
participant object is guarded to prevent that.

Signed-off-by: Daniel Calviño Sánchez <danxuliu@gmail.com>

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

@@ -2361,7 +2361,7 @@ public class CallActivity extends CallBaseActivity {
         String userId = "";
         if (hasMCU) {
             userId = webSocketClient.getUserIdForSession(session);
-        } else if (participantMap.get(session).getCalculatedActorType() == Participant.ActorType.USERS) {
+        } else if (participantMap.get(session) != null && participantMap.get(session).getCalculatedActorType() == Participant.ActorType.USERS) {
             userId = participantMap.get(session).getCalculatedActorId();
         }