Don't use wxApp-dependent functions in headless

When running headless, e.g. from Python script, the wxApp has not be
instantiated, so we cannot use wxConfig.  In this case, we fall back to
the class defaults in ADVANCED_CONFIG()

common/advanced_config.cpp

@@ -26,6 +26,7 @@
 #include <config_params.h>
 #include <settings/settings_manager.h>
 
+#include <wx/app.h>
 #include <wx/config.h>
 #include <wx/filename.h>
 #include <wx/log.h>
@@ -279,6 +280,10 @@ void ADVANCED_CFG::loadFromConfigFile()
 {
     const wxFileName k_advanced = getAdvancedCfgFilename();
 
+    // If we are running headless, use the class defaults because we cannot instantiate wxConfig
+    if( !wxTheApp )
+        return;
+
     if( !k_advanced.FileExists() )
     {
         wxLogTrace( AdvancedConfigMask, "File does not exist %s", k_advanced.GetFullPath() );