DRC dialog: fix a crash when clicking on a item in Footprint Warning panel:

if the warning is a missing footprint, obviously there is no marker associated.
It creates a crash due to a null pointer.

Fixes #4114
https://gitlab.com/kicad/code/kicad/issues/4114

common/rc_item.cpp

@@ -74,7 +74,14 @@ KIID RC_TREE_MODEL::ToUUID( wxDataViewItem aItem )
 
         switch( node->m_Type )
         {
-        case RC_TREE_NODE::MARKER:    return rc_item->GetParent()->GetUUID();
+        case RC_TREE_NODE::MARKER:
+            // rc_item->GetParent() can be null, if the parent is not existing
+            // when a RC item has no corresponding ERC/DRC marker
+            if( rc_item->GetParent() )
+                return rc_item->GetParent()->GetUUID();
+
+            break;
+
         case RC_TREE_NODE::MAIN_ITEM: return rc_item->GetMainItemID();
         case RC_TREE_NODE::AUX_ITEM:  return rc_item->GetAuxItemID();
         }