Fail cleanly when sending while disconnected

tests/test_send_receive.py

@@ -248,3 +248,14 @@ async def test_send_packet_deliver_failure(app, tx_packet):  # noqa: F811
             await app.send_packet(tx_packet)
 
     assert "Failed to deliver" in str(e)
+
+
+async def test_send_packet_disconnected(app, tx_packet):  # noqa: F811
+    """Sending after the radio was disconnected fails cleanly."""
+    app._api = None
+
+    with pytest.raises(zigpy.exceptions.DeliveryError) as e:
+        await app.send_packet(tx_packet)
+
+    assert "not connected" in str(e)
+    assert not app._pending_requests

zigpy_deconz/zigbee/application.py

@@ -565,6 +565,11 @@ async def send_packet(self, packet):
             tx_options |= t.DeconzTransmitOptions.USE_APS_ACKS
 
         async with self._limit_concurrency(priority=packet.priority):
+            # The radio can be disconnected while the request waits its turn, e.g.
+            # when a device is replied to after startup has failed
+            if self._api is None:
+                raise zigpy.exceptions.DeliveryError("Coordinator is not connected")
+
             req_id = self.get_sequence()
 
             if req_id in self._pending_requests: