TextField throw StringOutOfBoundException when using some input method

When using some specific input method,TextField will throw StringOutOfBoundException when editing.
You can download the IM from this link http://wbsoft.wn51.com/wnwb_800_50.exe(this is offical site and there will be no virus).
This IM is widely used in China.
We don't know when installing on non simplified Chinese edition of Windows,whether it can be reproduced.
After install the IM,goto a TextField and switch to the IM,and input "yiersan "(please note there is some spaces after "yiersan" to make sure all Chinese text go into the TextField,
indeed when using the IM for the first time,you need to type 3 spaces,for the next time,you only need to type 1 space.Anyway,type spaces until the IM dialog disappear and all character go into the TextField).
Then use backspace to delete characters until the last Chinese charater is deleted("until" means,there maybe some spaces after the last Chinese character,if you stroke too many spaces when input the characters)
It is important to delete only the last character.If you delete all character,there will be no exception.
After deleting the last character,then type any key ,such as "a",the exception will be thrown.