So when in Shell Defense it says " the only action you can take is a bonus action to emerge from your shell." 

So does that mean even bonus actions? Bonus Action moving  a "Flaming Sphere" for example.  

Bonus actions are (special) actions, as the very same text you quoted shows. This is also mentioned in the rules for bonus actions:

Bonus Actions

Various class features, spells, and other abilities let you take an additional action on your turn called a bonus action.

Edit, never mind, I followed that first link to the twitter. Bonus action is an action just more limited. 

Right, I knew that. The ability says "the only action you can take is a bonus action to emerge from your shell."  

So does "The only Action" mean I cant take other bonus actions as well. Say like bonus action Second wind. 

or does it mean All actions, bonus action cant be used BUT "emerge from your shell" 

That said, I believe you could start your turn with a bonus action to emerge from your shell and then use your regular action/move normally.