avoid null exceptions

bigbluebutton-client/src/org/bigbluebutton/modules/chat/views/ChatView.mxml

@@ -284,7 +284,10 @@ with BigBlueButton; if not, see <http://www.gnu.org/licenses/>.
       }
       
       private function getTabIndexFor(chatBox:ChatTab):int {
-        return chatTabs.getChildIndex(chatBox);
+        if (chatBox)
+          return chatTabs.getChildIndex(chatBox);
+        else
+          return -1;
       }
       
       private function onTabNavChange():void{

bigbluebutton-client/src/org/bigbluebutton/modules/screenshare/services/ScreenshareService.as

@@ -73,7 +73,7 @@ package org.bigbluebutton.modules.screenshare.services {
         }
         
         public function disconnect():void {
-            conn.disconnect();
+            if (conn) conn.disconnect();
         }
         
         public function checkIfPresenterIsSharingScreen():void {