Prevent warning when no printer is connected.

CURA-1036

resources/qml/MonitorButton.qml

@@ -109,7 +109,7 @@ Rectangle
         id: abortButton
 
         visible: printerConnected
-        enabled: Cura.MachineManager.printerOutputDevices[0].jobState == "paused" || Cura.MachineManager.printerOutputDevices[0].jobState == "printing"
+        enabled: printerConnected && Cura.MachineManager.printerOutputDevices[0].jobState == "paused" || Cura.MachineManager.printerOutputDevices[0].jobState == "printing"
 
         height: UM.Theme.getSize("save_button_save_to_button").height
         anchors.top: progressBar.bottom
@@ -156,7 +156,7 @@ Rectangle
         id: pauseButton
 
         visible: printerConnected
-        enabled: Cura.MachineManager.printerOutputDevices[0].jobState == "paused" || Cura.MachineManager.printerOutputDevices[0].jobState == "printing"
+        enabled: printerConnected && Cura.MachineManager.printerOutputDevices[0].jobState == "paused" || Cura.MachineManager.printerOutputDevices[0].jobState == "printing"
 
         height: UM.Theme.getSize("save_button_save_to_button").height
         anchors.top: progressBar.bottom