do not crash polling when just unsubscribed

ethers/providers/jsonrpc/subscriptions.nim

@@ -171,13 +171,11 @@ proc new*(_: type JsonRpcSubscriptions,
       discard
 
   proc getChanges(id: JsonNode): Future[JsonNode] {.async: (raises: [CancelledError]).} =
+    if mappedId =? subscriptions.subscriptionMapping.?[id]:
       try:
-      let mappedId = subscriptions.subscriptionMapping[id]
         let changes = await subscriptions.client.eth_getFilterChanges(mappedId)
         if changes.kind == JArray:
           return changes
-    except KeyError as error:
-      raiseAssert "subscription mapping invalid: " & error.msg
       except JsonRpcError:
         await resubscribe(id)
         # TODO: we could still miss some events between losing the subscription

nim.cfg

@@ -1,3 +1,4 @@
 -d:"chronicles_log_level=INFO"
 -d:"json_rpc_websocket_package=websock"
 --warning[LockLevel]:off
+--warning[DotLikeOps]:off