Fix an issue in the action panel.

When the panel is created while the user is in the preview stage, the
row has no width.

Contributes to CURA-5786.

resources/qml/ActionPanel/OutputProcessWidget.qml

@@ -99,6 +99,7 @@ Column
     {
         id: buttonRow
         spacing: UM.Theme.getSize("default_margin").width
+        width: parent.width
 
         Cura.ActionButton
         {