-wxColour *g_systemBtnFaceColour = NULL;
-wxColour *g_systemBtnShadowColour = NULL;
-wxColour *g_systemBtnHighlightColour = NULL;
-wxColour *g_systemHighlightColour = NULL;
+wxColour *g_systemBtnFaceColour = (wxColour *) NULL;
+wxColour *g_systemBtnShadowColour = (wxColour *) NULL;
+wxColour *g_systemBtnHighlightColour = (wxColour *) NULL;
+wxColour *g_systemHighlightColour = (wxColour *) NULL;