Fix crash in the splitter sample due to replacing null window

The sample could crash after the following sequence of actions:

1. Resize splitter to hide the right/left window.
2. Activate replace menu action.

Fix this by checking if the window being replaced is non-null.

Closes https://github.com/wxWidgets/wxWidgets/pull/1672

samples/splitter/splitter.cpp

@@ -448,8 +448,10 @@ void MyFrame::OnReplace(wxCommandEvent& WXUNUSED(event) )
 {
     if (m_replacewindow == NULL) {
         m_replacewindow = m_splitter->GetWindow2();
+        if (m_replacewindow != NULL) {
             m_splitter->ReplaceWindow(m_replacewindow, new wxPanel(m_splitter, wxID_ANY));
             m_replacewindow->Hide();
+        }
     } else {
         wxWindow *empty = m_splitter->GetWindow2();
         wxASSERT(empty != m_replacewindow);