object reference not set to an instance of an object

Unity Engine
1

So I am new with unity and i have a 2D project (Space Invader) and I have to use data structure in coding so I made a script and called it WaveSpawner which spawn a wave in the start of the game and I used linked list to link them but when I use " WaveSpawner.Wave.aliens.Remove(gameObject) " in Alien.sc which is a script component of prefabs of the enemy it gives me this error " object reference not set to an instance of an object ".
how to fix it?

5306640–533274–SpaceInvader.zip (532 Bytes)

2

One of the objects referenced is not allocated to (or set to a value). Either WaveSpawner, Wave or aliens.
You can split that into multiple lines to figure out which one, or use asserts before the line and when you find which one, make sure you SET it’s value before using it.

Assert(WaveSpawner);
Assert(WaveSpawner.Wave);
Assert(WaveSpawner.Wave.aliens);