have at least a measuring context available

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@60178 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/osx/carbon/dcscreen.cpp

@@ -31,8 +31,8 @@ wxScreenDCImpl::wxScreenDCImpl( wxDC *owner ) :
    wxWindowDCImpl( owner )
 {
 #if wxOSX_USE_COCOA_OR_IPHONE
-    m_graphicContext = NULL;
-    m_ok = false ;
+    SetGraphicsContext( wxGraphicsContext::Create() );
+    m_ok = true ;
 #else
     CGRect cgbounds ;
     cgbounds = CGDisplayBounds(CGMainDisplayID());