In ctext, sometimes we can get the error when using ctext::enableComments :
    can't read "length": no such variable
    can't read "length": no such variable
        while executing
    "$win index "$index + $length chars""
        (procedure "ctext::comments" line 26)
        invoked from within
    "ctext::comments .client.right.text 1"
        ("after" script)

This is because the length variable is not verified if defined before use.

Patch file for ctext.tcl file is :
--- ctext.tcl	2013-03-26 02:46:44.000000000 +0100
+++ ctext.tcl.new	2020-04-22 22:42:55.163217426 +0200
@@ -670,6 +670,10 @@
     while 1 {
 	set index [$win search -count length -regexp $commentRE $startIndex end]

+	if {![info exists length]} {
+	    break
+        }
+
 	if {$index == ""} {
 	    break
 	}