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.

(cherry picked from commit 009efa195b)

src/CutAndPaste.cpp

@@ -835,7 +835,10 @@ 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

status.23x

@@ -103,6 +103,9 @@ What's new
 
 - Fix wrap-around of spellchecker (#10383).
 
+- Avoid pointless 'undefined flex inset' errors when changing document
+  settings (bug 12163).
+
 
 * INTERNALS