[layouts] Avoid a crash when no pages exist in layout

Author: nyalldawson

src/core/layout/qgslayoutpagecollection.cpp

@@ -164,6 +164,9 @@ int QgsLayoutPageCollection::pageNumberForPoint( QPointF point ) const
 
 int QgsLayoutPageCollection::predictPageNumberForPoint( QPointF point ) const
 {
+  if ( mPages.empty() )
+    return 0;
+
   int pageNumber = 0;
   double startNextPageY = 0;
   Q_FOREACH ( QgsLayoutItemPage *page, mPages )

tests/src/python/test_qgslayoutpagecollection.py

@@ -573,6 +573,9 @@ def testPredictionPageNumberForPoint(self):
         l = QgsLayout(p)
         collection = l.pageCollection()
 
+        # no crash if no pages
+        self.assertEqual(collection.predictPageNumberForPoint(QPointF(1, 1)), 0)
+
         # add a page
         page = QgsLayoutItemPage(l)
         page.setPageSize(QgsLayoutSize(100, 100))