Help Server DarkOrbit

[officialhunterman]

Hellow i developed my own darkorbit files i fixed all the server and it is 2014 Updated the Problem is when i want to register it dosen´t let me register appears these:

 Spoiler

Notice: Trying to get property of non-object in C:\xampp\htdocs\index.php on line 47

Then i go to line 47 and appears these:

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if($MySQLi->query('SELECT null FROM users WHERE Name = \'' .$_POST['signup_username'] . '\'')->num_rows > 0):

I Don´t Know what to edit from the line 47 or to replace can someone help.