python: Fix Phoenix app setting

Phoenix doesn't have the same initialization as wxpython3 and so the
namespace doesn't get the wxApp() initialized to the existing instance.
In python, this is worked around by starting a new wxApp.
Unfortunately, this appears to overwrite the existing global instance
variable.  The issue _appears_ to be in Phoenix but for now we work
around it by saving and resetting the instance pointer in the main app.
The downside is that Python likely won't be able to respond to events
from C++

Fixes: lp:1809913
* https://bugs.launchpad.net/kicad/+bug/1809913

pcbnew/swig/python_scripting.cpp

@@ -366,9 +366,16 @@ wxWindow* CreatePythonShellWindow( wxWindow* parent, const wxString& aFramenameI
     PyDict_SetItemString( globals, "__builtins__", builtins );
     Py_DECREF( builtins );
 
+    auto app_ptr = wxTheApp;
     // Execute the code to make the makeWindow function we defined above
     PyObject*   result = PyRun_String( pcbnew_pyshell_one_step.str().c_str(), Py_file_input, globals, globals );
 
+#ifdef KICAD_SCRIPTING_WXPYTHON_PHOENIX
+    // This absolutely ugly hack is to work around the pyshell re-writing the global
+    // wxApp variable.  Once we can initialize Phoenix to access the appropriate instance
+    // of wx, we can remove this line
+    wxApp::SetInstance( app_ptr );
+#endif
     // Was there an exception?
     if( !result )
     {