Traversal on connect should be done on the main thread

Summary:
^

Reference: https://fb.workplace.com/groups/flippersupport/permalink/1775903596223624/

Reviewed By: LukeDefeo

Differential Revision: D53471045

fbshipit-source-id: 551caca8b5210f8a5117d0dbffae800e46a93af3

iOS/Plugins/FlipperKitUIDebuggerPlugin/FlipperKitUIDebuggerPlugin/Observer/UIDTreeObserverManager.m

@@ -15,6 +15,7 @@
 #import "UIDFrameScanEvent.h"
 #import "UIDInitEvent.h"
 #import "UIDJSONSerializer.h"
+#import "UIDMainThread.h"
 #import "UIDMetadataRegister.h"
 #import "UIDMetadataUpdateEvent.h"
 #import "UIDPerfStatsEvent.h"
@@ -170,12 +171,19 @@ - (void)startWithContext:(UIDContext*)context {
                                                         root:root
                                                 reportResult:reportResult];
       }];
-  UIApplication* application = _context.application;
-  if (application) {
-    [_rootObserver processNode:application
-                  withSnapshot:YES
-                   withContext:_context];
-  }
+
+  UIDRunBlockOnMainThreadAsync(^{
+    UIDTreeObserverManager* strongSelf = weakSelf;
+    if (strongSelf == nil) {
+      return;
+    }
+    UIApplication* application = strongSelf->_context.application;
+    if (application) {
+      [strongSelf->_rootObserver processNode:application
+                                withSnapshot:YES
+                                 withContext:strongSelf->_context];
+    }
+  });
 }
 
 - (void)stop {