Fix adding pages without associated window to wxTreebook

wxTreebook is supposed to allow not specifying any valid window for the
top-level pages, but this didn't work any longer, probably since the
changes of 02a92e23f3 (see #4379), as a
possibly null page was dereferenced without checking, resulting in a
crash.

Fix this by adding a missing check.

Also rename DoGetNonNullPage() to TryGetNonNullPage() to make it more
clear that this function can return null and add a unit test checking
that calling AddPage(NULL) really works (or at least doesn't crash).

See https://github.com/wxWidgets/wxWidgets/pull/921

include/wx/bookctrl.h

@@ -293,8 +293,9 @@ protected:
     // For classes that allow null pages, we also need a way to find the
     // closest non-NULL page corresponding to the given index, e.g. the first
     // leaf item in wxTreebook tree and this method must be overridden to
-    // return it if AllowNullPage() is overridden.
-    virtual wxWindow *DoGetNonNullPage(size_t page) { return m_pages[page]; }
+    // return it if AllowNullPage() is overridden. Note that it can still
+    // return null if there are no valid pages after this one.
+    virtual wxWindow *TryGetNonNullPage(size_t page) { return m_pages[page]; }
 
     // Remove the page and return a pointer to it.
     //