Catch "! Incomplete \\if[x]" error

Fixes: #10666

src/LaTeX.cpp

@@ -778,6 +778,12 @@ int LaTeX::scanLogFile(TeXErrors & terr)
 				}
 			}
 
+			if (prefixIs(token, "! Incomplete \\if")) {
+				// bug 10666. At this point its not clear we finish with error.
+				wait_for_error = desc;
+				continue;
+			}
+
 			if (prefixIs(token, "! Paragraph ended before \\Hy@setref@link was complete.")){
 					// bug 7344. We must rerun LaTeX if hyperref has been toggled.
 					retval |= ERROR_RERUN;

status.22x

@@ -47,6 +47,9 @@ What's new
 - Assure properly nested \begin{lang} and \end{lang} tags even when
   no language package is selected (bug 10685).
 
+- Catch "! Incomplete \if[x]" LaTeX error (bug 10666).
+
+
 * LYX2LYX