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

tests/controls/treebooktest.cpp

@@ -43,11 +43,13 @@ private:
         wxBOOK_CTRL_BASE_TESTS();
         CPPUNIT_TEST( Image );
         CPPUNIT_TEST( SubPages );
+        CPPUNIT_TEST( ContainerPage );
         CPPUNIT_TEST( Expand );
         CPPUNIT_TEST( Delete );
     CPPUNIT_TEST_SUITE_END();
 
     void SubPages();
+    void ContainerPage();
     void Expand();
     void Delete();
 
@@ -92,6 +94,20 @@ void TreebookTestCase::SubPages()
     CPPUNIT_ASSERT_EQUAL(3, m_treebook->GetPageParent(5));
 }
 
+void TreebookTestCase::ContainerPage()
+{
+    // Get rid of the pages added in setUp().
+    m_treebook->DeleteAllPages();
+    CHECK( m_treebook->GetPageCount() == 0 );
+
+    // Adding a page without the associated window should be allowed.
+    REQUIRE_NOTHROW( m_treebook->AddPage(NULL, "Container page") );
+    CHECK( m_treebook->GetPageParent(0) == -1 );
+
+    m_treebook->AddSubPage(new wxPanel(m_treebook), "Child page");
+    CHECK( m_treebook->GetPageParent(1) == 0 );
+}
+
 void TreebookTestCase::Expand()
 {
     wxPanel* subpanel1 = new wxPanel(m_treebook);