Add a missing check if mPtr is empty (bug #4715)

CHANGELOG.md

@@ -5,6 +5,7 @@
     Bug #4540: Rain delay when exiting water
     Bug #4701: PrisonMarker record is not hardcoded like other markers
     Bug #4714: Crash upon game load in the repair menu while the "Your repair failed!" message is active
+    Bug #4715: "Cannot get class of an empty object" exception after pressing ESC in the dialogue mode
     Feature #2229: Improve pathfinding AI
     Feature #3442: Default values for fallbacks from ini file
     Feature #4673: Weapon sheathing

apps/openmw/mwgui/dialogue.cpp

@@ -740,6 +740,9 @@ namespace MWGui
 
     bool DialogueWindow::isCompanion(const MWWorld::Ptr& actor)
     {
+        if (actor.isEmpty())
+            return false;
+
         return !actor.getClass().getScript(actor).empty()
                 && actor.getRefData().getLocals().getIntVar(actor.getClass().getScript(actor), "companion");
     }