qt5: Simplify code that creates an invalid Ref
diff --git a/qt5/src/poppler-annotation.cc b/qt5/src/poppler-annotation.cc
index c279840..5f23782 100644
--- a/qt5/src/poppler-annotation.cc
+++ b/qt5/src/poppler-annotation.cc
@@ -769,8 +769,7 @@
 {
     if (pdfAnnot == nullptr)
     {
-        const Ref invalid_ref = { -1, -1 };
-        return invalid_ref;
+        return Ref::INVALID();
     }
 
     return pdfAnnot->getRef();