fix NPE

app/src/main/java/chat/rocket/android/activity/MainActivity.java

@@ -164,7 +164,7 @@ public class MainActivity extends AbstractAuthedActivity {
   protected void onRoomIdUpdated() {
     super.onRoomIdUpdated();
 
-    if (roomId != null) {
+    if (roomId != null && RoomFragment.canCreate(RealmStore.get(serverConfigId))) {
       showFragment(RoomFragment.create(serverConfigId, roomId));
       closeSidebarIfNeeded();
     } else {

app/src/main/java/chat/rocket/android/fragment/chatroom/RoomFragment.java

@@ -67,6 +67,14 @@ public class RoomFragment extends AbstractChatRoomFragment
   public RoomFragment() {
   }
 
+  public static boolean canCreate(RealmHelper realmHelper) {
+    User currentUser = realmHelper.executeTransactionForRead(realm ->
+        User.queryCurrentUser(realm).findFirst());
+    Session session = realmHelper.executeTransactionForRead(realm ->
+        Session.queryDefaultSession(realm).findFirst());
+    return currentUser != null && session != null;
+  }
+
   /**
    * create fragment with roomId.
    */