[LyX/master] Fix assertion when canceling file dialog

[Enrico Forestieri]

commit 1cbcb303536800d889a45e8b1f286594987a6686
Author: Enrico Forestieri <forenr at lyx.org>
Date:   Sat Sep 2 18:13:31 2023 +0200

    Fix assertion when canceling file dialog
    
    Without this patch, when a file has to be loaded (for example,
    after "Document->Settings->Local Class" or "Insert->Graphics")
    and the file dialog is canceled, both Qt5 and Qt6 assert:
    ASSERT failure in QList::at: "index out of range"
---
 src/frontends/qt/FileDialog.cpp |    3 ++-
 1 files changed, 2 insertions(+), 1 deletions(-)

diff --git a/src/frontends/qt/FileDialog.cpp b/src/frontends/qt/FileDialog.cpp
index 704c34c..be99539 100644
--- a/src/frontends/qt/FileDialog.cpp
+++ b/src/frontends/qt/FileDialog.cpp
@@ -137,7 +137,8 @@ FileDialog::Result FileDialog::open(QString const & path,
 	FileDialog::Result result;
 	FileDialog::Results results = openMulti(path, filters, suggested, false);
 	result.first = results.first;
-	result.second = results.second.at(0);
+	if (result.first != FileDialog::Later)
+		result.second = results.second.at(0);
 	return result;
 }