Fix using wxMemoryDC without a GUI wxApp instance

This used to work, at least in wxMSW, but stopped working after the
(perfectly valid, on their own) changes of 2508efdd6e (Initialize
wxMemoryDC with a default font, 2019-08-13), as this resulted in calling
wxApp::GetTopWindow() that can only be called from the GUI code.

Fix this by adding wxApp::GetGUIInstance() and using it in
GetMainTopWindow(), so that we only call GetTopWindow() if we actually
have a GUI wxApp object on which to call it.

Implement this in terms of a new virtual IsGUI() which seems slightly
better than, although roughly equivalent to, using wxDynamicCast().

Closes https://github.com/wxWidgets/wxWidgets/pull/2617

Closes #19343.

include/wx/app.h

@@ -462,6 +462,9 @@ public:
     static wxAppConsole *GetInstance() { return ms_appInstance; }
     static void SetInstance(wxAppConsole *app) { ms_appInstance = app; }
 
+    // returns true for GUI wxApp subclasses
+    virtual bool IsGUI() const { return false; }
+
 
     // command line arguments (public for backwards compatibility)
     int argc;
@@ -684,6 +687,19 @@ public:
     // deactivated
     virtual void SetActive(bool isActive, wxWindow *lastFocus);
 
+    virtual bool IsGUI() const wxOVERRIDE { return true; }
+
+    // returns non-null pointer only if we have a GUI application object: this
+    // is only useful in the rare cases when the same code can be used in both
+    // console and GUI applications, but needs to use GUI-specific functions if
+    // the GUI is available
+    static wxAppBase *GetGUIInstance()
+    {
+        return ms_appInstance && ms_appInstance->IsGUI()
+                ? static_cast<wxAppBase*>(ms_appInstance)
+                : NULL;
+    }
+
 protected:
     // override base class method to use GUI traits
     virtual wxAppTraits *CreateTraits() wxOVERRIDE;
@@ -761,7 +777,7 @@ protected:
 // return the object of the correct type (i.e. MyApp and not wxApp)
 //
 // the cast is safe as in GUI build we only use wxApp, not wxAppConsole, and in
-// console mode it does nothing at all
+// console mode it does nothing at all (but see also wxApp::GetGUIInstance())
 #define wxTheApp static_cast<wxApp*>(wxApp::GetInstance())
 
 // ----------------------------------------------------------------------------

interface/wx/app.h

@@ -251,7 +251,7 @@ public:
         Returns the one and only global application object.
         Usually ::wxTheApp is used instead.
 
-        @see SetInstance()
+        @see SetInstance(), wxApp::GetGUIInstance()
     */
     static wxAppConsole* GetInstance();
 
@@ -838,6 +838,35 @@ public:
     */
     virtual wxVideoMode GetDisplayMode() const;
 
+    /**
+        Returns the current GUI wxApp object if any or @NULL otherwise.
+
+        This function should only be used in the rare cases when the same code
+        needs to work in both console and GUI applications, but needs to use
+        GUI-specific functionality if it is available, and so just calling
+        wxAppConsole::GetInstance() is insufficient while using ::wxTheApp is
+        incorrect, as the application object is not always a GUI wxApp.
+
+        For example:
+        @code
+            WXWidget handle = 0;
+            if ( wxApp* const app = wxApp::GetGUIInstance() ) {
+                if ( wxWindow* const w = app->GetTopWindow() ) {
+                    handle = w->GetHandle();
+                }
+            }
+            //else: no window to use
+
+            some_native_function_taking_a_window_handle(handle);
+        @endcode
+
+        Note that in this particular example, you could  use GetMainTopWindow()
+        which already does the same thing instead of doing it yourself.
+
+        @since 3.1.6
+    */
+    static wxAppConsole* GetGUIInstance();
+
     /**
         Returns @true if the application will exit when the top-level frame is deleted.
 
@@ -1213,7 +1242,9 @@ public:
 /**
     The global pointer to the singleton wxApp object.
 
-    @see wxApp::GetInstance()
+    This pointer can only be used in the GUI applications.
+
+    @see wxAppConsole::GetInstance(), wxApp::GetGUIInstance()
 */
 wxApp *wxTheApp;
 

src/common/appcmn.cpp

@@ -193,7 +193,7 @@ wxWindow* wxAppBase::GetTopWindow() const
 /* static */
 wxWindow* wxAppBase::GetMainTopWindow()
 {
-    const wxAppBase* const app = static_cast<wxAppBase*>(GetInstance());
+    const wxAppBase* const app = GetGUIInstance();
 
     return app ? app->GetTopWindow() : NULL;
 }