MainRootWindow.qml: PR CustomMainRootWindow: fix bug, empty loader visible as blank panel:

when clicking back button screen from any tool, it would go blank and
then it would collapse, because the loader was being emptied before the
drawer containing it closed completely

src/ui/MainRootWindow.qml

@@ -436,6 +436,11 @@ ApplicationWindow {
         property alias toolSource:  toolDrawerLoader.source
         property alias toolIcon:    toolIcon.source
 
+        // Unload the loader only after closed, otherwise we will see a "blank" loader in the meantime
+        onClosed: {
+            toolDrawer.toolSource = ""
+        }
+        
         Rectangle {
             id:             toolDrawerToolbar
             anchors.left:   parent.left
@@ -492,7 +497,6 @@ ApplicationWindow {
                 width:              (backTextLabel.x + backTextLabel.width) - backIcon.x
                 onClicked: {
                     toolDrawer.visible      = false
-                    toolDrawer.toolSource   = ""
                 }
             }
         }