prevent panic when handling an LSP response with no request (#2475)

A language server may push a response which doesn't belong to any
request. With this change, we discard the response rather than
crashing.

In the case of #2474, the language server sends an error message
with a null request ID which should not ever exist in the
`pending_requests` HashMap.

closes #2474

helix-lsp/src/transport.rs

@@ -215,13 +215,7 @@ impl Transport {
             }
         };
 
-        let tx = self
+        if let Some(tx) = self.pending_requests.lock().await.remove(&id) {
-            .pending_requests
-            .lock()
-            .await
-            .remove(&id)
-            .expect("pending_request with id not found!");
-
             match tx.send(result).await {
                 Ok(_) => (),
                 Err(_) => error!(
@@ -229,6 +223,13 @@ impl Transport {
                     id
                 ),
             };
+        } else {
+            log::error!(
+                "Discarding Language Server response without a request (id={:?}) {:?}",
+                id,
+                result
+            );
+        }
 
         Ok(())
     }