Fix notification about negative balance - Closes #477

[alepop]

Closes #477

[slaweet]

@alepop can you please add some description of how/why the bug happens? I don't see what difference do your changes make, given that JavaScript is single-threaded.

[alepop]

@slaweet I know that the JavaScript in single-threaded and so this bug confused me. I can not find an answer to this error except as a possible balance change after the check, but how I not understand. And I could not reproduce this behavior. I think that the variable cache may help.

[slaweet]

Even more explicit that negative balance cannot happen would be to do the const amount = ... outside the if and then do if(amount>0)

[slaweet]

thanks @alepop

[slaweet]

I still don't see how this issue could have happened, but let's merge this in and see if the issue happens again.