Ignore system colour change events in not yet initialized wxPropertyGrid.

Handling these events before the initialization was over could result in a
crash because m_categoryDefaultCell wasn't yet initialized when the window was
created. Fix this by ignoring these events during initialization.

Closes #15642.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_3_0_BRANCH@75128 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/propgrid/propgrid.cpp

@@ -1297,8 +1297,10 @@ void wxPropertyGrid::CalculateFontAndBitmapStuff( int vspacing )
 
 void wxPropertyGrid::OnSysColourChanged( wxSysColourChangedEvent &WXUNUSED(event) )
 {
+    if ((m_iFlags & wxPG_FL_INITIALIZED)!=0) {
         RegainColours();
         Refresh();
+    }
 }
 
 // -----------------------------------------------------------------------