Canceling the Office Importer dialog re-inserts the last imported text

Follow this steps to reproduce:

• edit a new page in WYSIWYG mode
• open the Office Importer dialog
• write some text in the text area (no need to paste)
• click "Import"
• press Enter to create a new line
• open the Office Importer dialog again
• don't write or paste anything, just click "Cancel" or the X button

As a result the previously inserted text is inserted again.