Dialog: updateLayoutParameters after calling syncToMainItemSize

updateLayoutParameters's uses the borders in order to set the max and
minimum size, and therefore syncToMainItemSize should always be called
before it is ever called.

Also added code to not do anything if we are not completely initialized.

src/plasmaquick/dialog.cpp

@@ -239,6 +239,9 @@ void DialogPrivate::updateVisibility(bool visible)
                 cachedGeometry = QRect();
             }
             syncToMainItemSize();
+            if (mainItemLayout) {
+                updateLayoutParameters();
+            }
         }
     }
 
@@ -424,7 +427,7 @@ void DialogPrivate::updateMaximumHeight()
 
 void DialogPrivate::updateLayoutParameters()
 {
-    if (!componentComplete) {
+    if (!componentComplete || !mainItem || !q->isVisible()) {
         return;
     }
     Q_ASSERT(mainItem);