Disable external program field when not selecting use external program

src/main/java/name/nathanmcrae/numbersstation/MainSettingsController.java

@@ -311,6 +311,8 @@ public class MainSettingsController {
                 };
             }
         });
+
+        externalProgramCommandField.disableProperty().bind(externalProgramRadioButton.selectedProperty().not());
     }
 
     @FXML