Fix "Cannot set property of null" error on textarea
Fix "Cannot set property of null" error on textarea

When the textarea on the download page is not available (when we have no
download page), an error was caused when we tried to get
`textarea.value`. Check for `textarea`'s existence before asking for its
value.

teddywing/chrome-copy-urls-from-all-tabs,teddywing/chrome-copy-urls-from-all-tabs