Reset wxTheApp when starting to destroy the object it points to.

Using wxTheApp when the corresponding object is already half destroyed can't
be a good idea so reset it to NULL as soon as possible.

Closes #11525.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@62813 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/common/appbase.cpp

@@ -154,6 +154,10 @@ wxAppConsoleBase::wxAppConsoleBase()
 
 wxAppConsoleBase::~wxAppConsoleBase()
 {
+    // we're being destroyed and using this object from now on may not work or
+    // even crash so don't leave dangling pointers to it
+    ms_appInstance = NULL;
+
     delete m_traits;
 }