Fix activation not working sometimes

The current player cell was only being updated when the reference was not empty, causing it
to incorrectly detect a cell change the first time something was activated in a newly visited cell, immediately closing the opened dialogue again.

apps/openmw/mwgui/referenceinterface.cpp

@@ -18,17 +18,17 @@ namespace MWGui
 
     void ReferenceInterface::checkReferenceAvailable()
     {
-        if (mPtr.isEmpty())
-            return;
-
         MWWorld::Ptr::CellStore* playerCell = MWBase::Environment::get().getWorld()->getPlayer().getPlayer().getCell();
 
         // check if player has changed cell, or count of the reference has become 0
         if ((playerCell != mCurrentPlayerCell && mCurrentPlayerCell != NULL)
-            || mPtr.getRefData().getCount() == 0)
+             || (!mPtr.isEmpty() && mPtr.getRefData().getCount() == 0))
         {
-            mPtr = MWWorld::Ptr();
-            onReferenceUnavailable();
+            if (!mPtr.isEmpty())
+            {
+                mPtr = MWWorld::Ptr();
+                onReferenceUnavailable();
+            }
         }
 
         mCurrentPlayerCell = playerCell;