Catch an error in etherswitch when ubridge die

Fix #907

gns3server/compute/dynamips/nodes/ethernet_switch.py

@@ -211,7 +211,8 @@ class EthernetSwitch(Device):
         nio = self._nios[port_number]
         if isinstance(nio, NIOUDP):
             self.manager.port_manager.release_udp_port(nio.lport, self._project)
-        yield from self._hypervisor.send('ethsw remove_nio "{name}" {nio}'.format(name=self._name, nio=nio))
+        if self._hypervisor:
+            yield from self._hypervisor.send('ethsw remove_nio "{name}" {nio}'.format(name=self._name, nio=nio))
 
         log.info('Ethernet switch "{name}" [{id}]: NIO {nio} removed from port {port}'.format(name=self._name,
                                                                                               id=self._id,