Show error if server is not ready when a tool run is requested

src/sidebar.ts

@@ -141,7 +141,12 @@ function runTool(uri: string, toolName: string, constants: { name: string; value
                     resolve(null);
                 }
             });
-            await client?.sendRequest<string>("modest/runTool", jsonObject, token);
+            if (client === null) {
+                vscode.window.showErrorMessage("Server not ready yet, try again later");
+                resolve(null);
+            } else {
+                await client?.sendRequest<string>("modest/runTool", jsonObject, token);
+            }
         });
     });
 }