Added check to see if selectedPrinter is set

CL-500

plugins/UM3NetworkPrinting/DiscoverUM3Action.qml

@@ -279,7 +279,7 @@ Cura.MachineAction
                     wrapMode: Text.WordWrap
                     text:{
                         // The property cluster size does not exist for older UM3 devices.
-                        if(base.selectedPrinter.clusterSize == null || base.selectedPrinter.clusterSize == 1)
+                        if(base.selectedPrinter != undefined && base.selectedPrinter.clusterSize == null || base.selectedPrinter.clusterSize == 1)
                         {
                             return "";
                         }