Initialize m_action_window to NULL in wxAuiManager ctor (fixes #10383).

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@58120 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/aui/framemanager.cpp

@@ -582,6 +582,7 @@ END_EVENT_TABLE()
 wxAuiManager::wxAuiManager(wxWindow* managed_wnd, unsigned int flags)
 {
     m_action = actionNone;
+    m_action_window = NULL;
     m_last_mouse_move = wxPoint();
     m_hover_button = NULL;
     m_art = new wxAuiDefaultDockArt;