Fix unexpected termination by clicking run button.

1) When TMP (default kidcode path) contains multi-byte character code, KidsRuby (Qt) fails to write kidcode.rb.

2) In addition, Qt::MessageBox::warning causes 'method_missing'.
Finally, KidsRuby is unexpectedly terminated by clicking Run button.

1) is addressed by force_encoding utf-8. 2) is addressed by passing nil instead of self.
Please review the code.