FIX: gui: fix the crash issue when close guide frame at the beginning

jira: STUDIO-9941
Change-Id: I47e189e838ae606d294d0c0b4ccffc6f833a27c3
(cherry picked from commit 093a7c25cb7507d0d6af228c2efa3410e1184c27)

src/slic3r/GUI/WebGuideDialog.hpp

@@ -110,6 +110,8 @@ private:
 
     //First Load
     bool bFirstComplete{false};
+    bool m_destroy{false};
+    boost::thread* m_load_task{ nullptr };
 
     // User Config
     bool PrivacyUse;