Fix bug where client plugins weren't always started

Summary:
Changelog: Fixed issue where occasionally a plugin wouldn't open after starting Flipper

This fixes a long standing issue where rarely Flipper wouldn't show the selected plugin. This turned out to be a raise condition, that was easy to reproduce in the Flipper browser version; if a client register before all the plugins are loaded, the plugins that are enabled for that client, but not loaded yet, will not instantiate and hence not show up. This diff fixes that

Reviewed By: timur-valiev, aigoncharov

Differential Revision: D32987162

fbshipit-source-id: f3179cd9b6f2e4e79d05be1f2236f63acdf50495

desktop/flipper-ui-core/src/utils/waitFor.tsx

@@ -0,0 +1,27 @@
+/**
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ *
+ * @format
+ */
+
+import {State, Store} from '../reducers/index';
+
+export async function waitFor(
+  store: Store,
+  predicate: (state: State) => boolean,
+): Promise<void> {
+  if (predicate(store.getState())) {
+    return;
+  }
+  return new Promise<void>((resolve) => {
+    const unsub = store.subscribe(() => {
+      if (predicate(store.getState())) {
+        unsub();
+        resolve();
+      }
+    });
+  });
+}