Repo to reproduce Flow issue.

The following program:

type obj = {
  a : number,
  b : number
}
type ty = {
  c : ?obj
}

function test(x : ty){
  if(x.c == null){
    throw new Error('Erorr!')
  }
  var temp = x.c.a;
  console.log('test')
  return x.c.b;
}

produces the following errors:

index.js:17
 17:   return x.c.b;
                  ^ property `b`. Property cannot be accessed on possibly null value
 17:   return x.c.b;
              ^^^ null

index.js:17
 17:   return x.c.b;
                  ^ property `b`. Property cannot be accessed on possibly undefined value
 17:   return x.c.b;
              ^^^ undefined

Removing the console.log eliminates the error. Also, x.c will definitely not be null given the thrown exception is in the then branch of the if statement.