Do not explicitly set selection to -1 when deleting all pages

Avoid calling wxNotebook::SetSelection(-1) when the notebook is cleared.

Closes https://github.com/wxWidgets/wxWidgets/pull/1254

src/qt/notebook.cpp

@@ -167,13 +167,22 @@ wxSize wxNotebook::CalcSizeFromPage(const wxSize& sizePage) const
 
 bool wxNotebook::DeleteAllPages()
 {
-    if ( !wxNotebookBase::DeleteAllPages() )
-        return false;
+    // Nothing to do if the notebook was not created yet,
+    // and return true just like other ports do.
+    if ( !m_qtTabWidget )
+        return true;
 
+    // Block signals to not receive selection changed updates
+    // which are sent by Qt after the selected page was deleted.
     m_qtTabWidget->blockSignals(true);
-    m_qtTabWidget->clear();
+
+    // Pages will be deleted one by one in the base class.
+    // There's no need to explicitly clear() the Qt control.
+    bool deleted = wxNotebookBase::DeleteAllPages();
+
     m_qtTabWidget->blockSignals(false);
-    return true;
+
+    return deleted;
 }
 
 int wxNotebook::SetSelection(size_t page)