[LyX/master] Avoid pointless 'undefined flex inset' errors

[Jean-Marc Lasgouttes]

commit 009efa195bf2837a007c0b070261b34a715b9b39
Author: Jean-Marc Lasgouttes <lasgouttes at lyx.org>
Date:   Tue Mar 2 09:12:47 2021 +0100

    Avoid pointless 'undefined flex inset' errors
    
    When switching class, it does not make sense to flag flex insets that
    are undefined if they were already undefined before.
    
    Part of bug #12163.
---
 src/CutAndPaste.cpp |    5 ++++-
 1 files changed, 4 insertions(+), 1 deletions(-)

diff --git a/src/CutAndPaste.cpp b/src/CutAndPaste.cpp
index 6ab73f4..4a93966 100644
--- a/src/CutAndPaste.cpp
+++ b/src/CutAndPaste.cpp
@@ -853,7 +853,10 @@ void switchBetweenClasses(DocumentClassConstPtr oldone,
 			docstring const & n = newone->insetLayout(layoutName).name();
 			bool const is_undefined = n.empty() ||
 				n == DocumentClass::plainInsetLayout().name();
-			if (!is_undefined)
+			docstring const & oldn = oldone->insetLayout(layoutName).name();
+			bool const was_undefined = oldn.empty() ||
+				oldn == DocumentClass::plainInsetLayout().name();
+			if (!is_undefined || was_undefined)
 				continue;
 
 			// The flex inset is undefined in newtc