Avoid crash with GTK3 if console program is linked to GUI library

A console program should not really be linked with the GUI library, but it used
to work, and it's unavoidable with the monolithic build, so fix the regression.
See #23981

src/gtk/settings.cpp

@@ -13,6 +13,7 @@
 #include "wx/settings.h"
 
 #ifndef WX_PRECOMP
+    #include "wx/app.h"
     #include "wx/toplevel.h"
     #include "wx/module.h"
 #endif
@@ -1206,6 +1207,10 @@ bool wxSystemSettingsModule::OnInit()
 
     m_proxy = nullptr;
 
+    wxAppConsole* app = wxAppConsole::GetInstance();
+    if (!app || !app->IsGUI())
+        return true;
+
     // GTK_THEME environment variable overrides other settings
     if (getenv("GTK_THEME") == nullptr)
     {