[macOS] UI process sometimes crashes under ViewGestureController::disconnectFromProcess()

https://bugs.webkit.org/show_bug.cgi?id=287493
rdar://143899055

Reviewed by Charlie Wolfe and Abrar Rahman Protyasha.

Null check `m_mainFrameProcess` before attempting to remove the `ViewGestureController` message
receiver. In the case where the main `WebProcessProxy` outlives the controller, `WebProcessProxy`
teardown should already have cleaned up message receivers (and invoked any messages pending reply)
anyways, so it should be safe to avoid this removal step.

* Source/WebKit/UIProcess/ViewGestureController.cpp:
(WebKit::ViewGestureController::disconnectFromProcess):

Canonical link: https://commits.webkit.org/290239@main

Author: whsieh

Source/WebKit/UIProcess/ViewGestureController.cpp

@@ -112,8 +112,9 @@ void ViewGestureController::disconnectFromProcess()
     if (!m_isConnectedToProcess)
         return;
 
-    Ref { *m_mainFrameProcess }->removeMessageReceiver(Messages::ViewGestureController::messageReceiverName(), *m_webPageIDInMainFrameProcess);
-    m_mainFrameProcess = nullptr;
+    if (RefPtr mainFrameProcess = std::exchange(m_mainFrameProcess, nullptr).get())
+        mainFrameProcess->removeMessageReceiver(Messages::ViewGestureController::messageReceiverName(), *m_webPageIDInMainFrameProcess);
+
     m_webPageIDInMainFrameProcess = std::nullopt;
     m_isConnectedToProcess = false;
 }