Fix crash when cellular modem is disconnected.

Bug: 1001687
Change-Id: I50c527dcce3ffc5b831519c346ff0db30f5495a7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1790471
Commit-Queue: Jon Mann <jonmann@chromium.org>
Reviewed-by: Kyle Horimoto <khorimoto@chromium.org>
Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#694975}

chromeos/services/network_config/cros_network_config.cc

@@ -205,7 +205,14 @@ mojom::NetworkStatePropertiesPtr NetworkStateToMojo(
     // is not connectable.
     const DeviceState* device =
         network_state_handler->GetDeviceState(network->device_path());
-    DCHECK(device) << "Device path not found: " << network->device_path();
+    if (!device) {
+      // When a device is removed (e.g. cellular modem unplugged) it's possible
+      // for the device object to disappear before networks on that device
+      // are cleaned up.  This fixes crbug/1001687.
+      NET_LOG(DEBUG) << "Cellular is not available.";
+      return nullptr;
+    }
+
     if (device->IsSimLocked() || device->scanning())
       result->connectable = false;
   }