Initialize wxGenericProgressDialog::m_winDisabler properly.

This field wasn't initialized by Init() which might have been harmless
considering the current code structure but still untidy, do set it to NULL
there.

Also move the forward declaration of wxWindowDisabler class in
wx/generic/progdlgg.h to the top of the file for consistency with the other
forward declarations.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@65498 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/generic/progdlgg.cpp

@@ -125,6 +125,8 @@ void wxGenericProgressDialog::Init(wxWindow *parent, int maximum, int style)
 
     m_hasAbortButton =
     m_hasSkipButton = false;
+
+    m_winDisabler = NULL;
 }
 
 wxGenericProgressDialog::wxGenericProgressDialog(wxWindow *parent,