Comment preview not cleared when adding new comments.

When adding a new comment, the comment box will show the previous comment if the previous comment was created while in preview mode.

Steps to reproduce

1. Create a new code review and add a file.
2. Click in the file to add a comment.
3. Type some random text.
4. Click the Preview Comment button to preview the formatted comment.
5. Click the Comment button to accept the comment while in preview mode.
6. Click on another line in the file to create a new comment.

At this point, the comment box will be in preview mode and will show the previously added comment. Clicking the Preview Comment button will return to an empty box.

Expected behavior

Creating the second comment will show an empty comment box in edit mode, not preview mode.