webgui: terminate application via timeout

Otherwise it could happen, that calling object (websocket handler) will
be destroyed by such action. 

Like now civetweb is blocked, waiting that websoocket request is
processed, but gApplication->Terminate() starts deleting of all
connections and trying to call mg_stop.

gui/canvaspainter/v7/src/TCanvasPainter.cxx

@@ -569,7 +569,7 @@ void ROOT::Experimental::TCanvasPainter::ProcessData(unsigned connid, const std:
       conn->fGetMenu = cdata;
    } else if (arg == "QUIT") {
       // use window manager to correctly terminate http server
-      TWebWindowsManager::Instance()->Terminate(0);
+      TWebWindowsManager::Instance()->Terminate();
       return;
    } else if (arg == "RELOAD") {
       conn->fSend = 0; // reset send version, causes new data sending

gui/webdisplay/inc/ROOT/TWebWindowsManager.hxx

@@ -66,7 +66,7 @@ public:
    int WaitFor(WebWindowWaitFunc_t check, double tm);
 
    /// Terminate http server and ROOT application
-   void Terminate(int code = 0);
+   void Terminate();
 
 };
 

gui/webdisplay/src/TWebWindowsManager.cxx

@@ -31,6 +31,7 @@
 #include "TString.h"
 #include "TStopwatch.h"
 #include "TApplication.h"
+#include "TTimer.h"
 
 /** \class ROOT::Experimental::TWebWindowManager
 \ingroup webdisplay
@@ -323,11 +324,12 @@ int ROOT::Experimental::TWebWindowsManager::WaitFor(WebWindowWaitFunc_t check, d
 //////////////////////////////////////////////////////////////////////////
 /// Terminate http server and ROOT application
 
-void ROOT::Experimental::TWebWindowsManager::Terminate(int code)
+void ROOT::Experimental::TWebWindowsManager::Terminate()
 {
    if (fServer)
       fServer->SetTerminate();
 
+   // use timer to avoid situation when calling object is deleted by terminate
    if (gApplication)
-      gApplication->Terminate(code);
+      TTimer::SingleShot(100, "TApplication", gApplication, "Terminate()");
 }