Avoid referencing parents after deletion

We look at the parent when deleting EDA_ITEMs but when clearing the test
sheet, we delete the sheet before the pin, causing a use-after-free.
Avoid this by setting parents to null in this case

Fixes https://gitlab.com/kicad/code/kicad/-/issues/17961

eeschema/dialogs/panel_eeschema_color_settings.cpp

@@ -106,7 +106,11 @@ PANEL_EESCHEMA_COLOR_SETTINGS::~PANEL_EESCHEMA_COLOR_SETTINGS()
     delete m_currentSettings;
 
     for( EDA_ITEM* item : m_previewItems )
+    {
+        // Avoid referencing items after they are deleted (we don't control order)
+        item->SetParent( nullptr );
         delete item;
+    }
 }