Check for consecutive value in Map? - oop

Question

I'm creating a chat app and don't want to display the user avatar over and over if a user sends multiple messages consecutively.
The user messages are stored in a Map, where the newest message has the index 0.
To check if a message at an index is sent by the same person as the message before, I use the following method:
bool _sameUser () {
if (index > 0 && map != null && map[index + 1] != null && map[index + 1]['fromUser’] == map[index][‘fromUser’]) {
return true;
} else {
return false;
}
}
However, this doesn't work for the newest message or if there are more than two messages from the same user.
How I can rewrite the conditional so it works as intended?
Thanks!
UPDATE:
After try #Marcel answer and test more I find another issue maybe cause this.
map[index + 1] != null is not true even when message 2 is send. Is only true after message 3 is send.
I test with conditional in function:
if (map[index + 1] != null) {
print('!=null');
}

Answer 1

There's no reason your code shouldn't work for the index 0 except you test index > 0.
By removing that, it should work fine:
bool _sameUser () {
assert(index >= 0);
assert(map != null);
return map[index + 1] != null && map[index + 1]['fromUser'] == map[index]['fromUser'];
}
Because I assumed, the index should never be smaller than 0 and the map should never be null, I moved some of the condition code to assert statements.
Also, because your if-expression is a boolean, you can just return it directly.