Fix some warnings

Contributes to CL-1152

plugins/UM3NetworkPrinting/resources/qml/MonitorConfigOverrideDialog.qml

@@ -54,6 +54,10 @@ UM.Dialog
         wrapMode: Text.WordWrap
         text:
         {
+            if (!printer.activePrintJob)
+            {
+                return ""
+            }
             var topLine
             if (materialsAreKnown(printer.activePrintJob))
             {

plugins/UM3NetworkPrinting/resources/qml/MonitorPrintJobPreview.qml

@@ -60,6 +60,10 @@ Item
         height: 0.5 * printJobPreview.height
         source:
         {
+            if (!printJob)
+            {
+                return ""
+            }
             if (printJob.configurationChanges.length > 0)
             {
                 return "../svg/warning-icon.svg"

plugins/UM3NetworkPrinting/resources/qml/MonitorPrintJobProgressBar.qml

@@ -55,7 +55,7 @@ Item
             left: progressBar.right
             leftMargin: 18 * screenScaleFactor // TODO: Theme!
         }
-        text: Math.round(printJob.progress * 100) + "%"
+        text: printJob ? Math.round(printJob.progress * 100) + "%" : "0%"
         color: printJob && printJob.isActive ? "#374355" : "#babac1" // TODO: Theme!
         width: contentWidth
         font: UM.Theme.getFont("medium") // 14pt, regular

plugins/UM3NetworkPrinting/resources/qml/PrintJobContextMenu.qml

@@ -182,7 +182,7 @@ Item {
                     abortConfirmationDialog.visible = true;
                     popup.close();
                 }
-                text: printJob.state == "aborting" ? catalog.i18nc("@label", "Aborting...") : catalog.i18nc("@label", "Abort");
+                text: printJob && printJob.state == "aborting" ? catalog.i18nc("@label", "Aborting...") : catalog.i18nc("@label", "Abort");
                 visible: {
                     if (!printJob) {
                         return false;