b88d5e08ce
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