Mirror window sensitivity fix

ubl-settings-update-mirror-add.glade

@@ -174,6 +174,7 @@ mirrorlist - mirrors file, make sure server URL is NOT included in this file! Si
                                         <property name="width-request">28</property>
                                         <property name="height-request">28</property>
                                         <property name="visible">True</property>
+                                        <property name="sensitive">False</property>
                                         <property name="can-focus">True</property>
                                         <property name="receives-default">True</property>
                                         <property name="tooltip-text" translatable="yes">Open file explorer</property>