C#: Fix crash due to missing gchandle ref null check

modules/mono/csharp_script.cpp

@@ -1189,7 +1189,7 @@ bool CSharpLanguage::refcount_decremented_instance_binding(Object *p_object) {
 		return refcount == 0;
 	Ref<MonoGCHandle> &gchandle = ((Map<Object *, CSharpScriptBinding>::Element *)data)->get().gchandle;
 
-	if (refcount == 1 && !gchandle->is_weak()) { // The managed side also holds a reference, hence 1 instead of 0
+	if (refcount == 1 && gchandle.is_valid() && !gchandle->is_weak()) { // The managed side also holds a reference, hence 1 instead of 0
 		// If owner owner is no longer referenced by the unmanaged side,
 		// the managed instance takes responsibility of deleting the owner when GCed.