problem in converting rich text to map

problem in converting rich text to map

hi
please do below items:
1- create a rich text field in a form
2- place a map into it
3- set a string variable equal value of rich text
4- convert string variable to map.
you see the result of stage 4 every time is {}.
if we use multi line text instead of rich text then this problem doesn't occurred.
please help me as soon as possible