Validation bug in TestHost.exe

[juliet86]

Program accepts any character as operation and shows no error massage.
Steps:

1. Open TestHost.exe
2. Type first number and push "Enter"
3. Type second number and push "Enter"
4. Type any character that is not +-*: (I typed special character "," and letter "g")
   Expected result: program doesn't accept any character that is not required and shows appropriate error massage.
   Actual result: program accepts any character (see attachment)