This letter cannot be read because it has been burnt to ash. You can only throw away the ashes. Make sure you read the letter and retain the contents in your mind, before you throw away the ashes, if you want to use them later.

But this is claiming that the letter does not exist, while allowing you to read the contents of the letter.

If it can show you the content, the container must exist!

In a post office, open a letter, burn the envelope. The content now exists, but the message does not.

but that implies the message is the envelope, which is not typically what a message means.