Fix binding loop warnings that would sometimes occur when editing Brand entry

Contributes to CURA-342

resources/qml/Preferences/MaterialsPage.qml

@@ -129,7 +129,7 @@ UM.ManagementPage
 
                 enabled: base.currentItem != null && !base.currentItem.readOnly
 
-                checkable: true
+                checkable: enabled
             }
         }
 
@@ -144,7 +144,7 @@ UM.ManagementPage
                 bottom: parent.bottom
             }
 
-            editingEnabled: base.currentItem != null && !base.currentItem.readOnly && editButton.checked;
+            editingEnabled: editButton.checkable && editButton.checked;
 
             properties: materialProperties
             containerId: base.currentItem != null ? base.currentItem.id : ""