setState on onPressed with null

Question

I am new to flutter. I have copy this code from bluetooth serial package example and I want to add a setState.

FlatButton(
 onPressed: isConnected ? () => 
 _sendMessage('0') : null
)

This is what I already tried:

FlatButton(
 onPressed:(){
  if(isConnected) 
   _sendMessage('0');
  setState(() {
   counter=true;
  }); 
  }
)

But the state didnt change.

What is the correct way on adding setState on this?