Qual forma está gramaticamente correta no Future Continuous?

Good morning friends! I've been studying the future continuous and, unfortunately, I have doubts . First, I want to know why we can't use the future continuous with time expressions, such as: while, when, after, before, as soon as etc. Take a look at these examples that I found on the Net:

While I am finishing my work, Tess is going to be preparing dinner. - CORRECT
While I will be finishing my work, Tess will be preparing dinner. - NOT CORRECT

Please guys, can you explain this? Besides, I wanna know which sentence bellow is correct according to the grammar .

Bernardo will be eating popcorn while Katy will be eating chocolate.
OR
Bernardo will be eating popcorn while Katy is eating chocolate.

Thanks for all,
Bernardo