[Ermine] Prevent potential toDouble call on null

This is to prevent behavior that caused fxb/41681

Change-Id: I6a54d111b1dd0e2f7e3f32a70ed351e9f5071e1f
diff --git a/session_shells/ermine/settings/lib/src/weather.dart b/session_shells/ermine/settings/lib/src/weather.dart
index 9b26dff..01a6b94 100644
--- a/session_shells/ermine/settings/lib/src/weather.dart
+++ b/session_shells/ermine/settings/lib/src/weather.dart
@@ -101,7 +101,7 @@
     var properties = data['properties'];
     location
       ..observation = properties['textDescription']
-      ..tempInDegrees = properties['temperature']['value'].toDouble();
+      ..tempInDegrees = properties['temperature']['value']?.toDouble();
   }
 
   // Read the string response from the [HttpClientResponse].