So I was trying to add lists to my CharacterController2D code to store some data. I was being forced to use ArrayList but wanted to use List so I added Systems.Collections.Generic to the top of the project. Then the IENumerator function I had from before but didn't need was throwing an error. It also greyed out the Systems.Collections at the top, since I had the Collections.Generic (I guess?) so I removed the Systems.Collections. After that, the List <> worked fine but when I ran the game I had these errors: NullReferenceException: Object reference not set to an instance of an object UnityEditor.Graphs.Edge.WakeUp () (at C:/buildslave/unity/build/Editor/Graphs/UnityEditor.Graphs/Edge.cs:114) UnityEditor.Graphs.Graph.DoWakeUpEdges (System.Collections.Generic.List`1[T] inEdges, System.Collections.Generic.List`1[T] ok, System.Collections.Generic.List`1[T] error, System.Boolean inEdgesUsedToBeValid) (at C:/buildslave/unity/build/Editor/Graphs/UnityEditor.Graphs/Graph.cs:387) UnityEditor.Graphs.Graph.WakeUpEdges (System.Boolean clearSlotEdges) (at C:/buildslave/unity/build/Editor/Graphs/UnityEditor.Graphs/Graph.cs:286) UnityEditor.Graphs.Graph.WakeUp (System.Boolean force) (at C:/buildslave/unity/build/Editor/Graphs/UnityEditor.Graphs/Graph.cs:272) UnityEditor.Graphs.Graph.WakeUp () (at C:/buildslave/unity/build/Editor/Graphs/UnityEditor.Graphs/Graph.cs:250) UnityEditor.Graphs.Graph.OnEnable () (at C:/buildslave/unity/build/Editor/Graphs/UnityEditor.Graphs/Graph.cs:245) I'm pretty new to coding and have only taken some Unity game courses online. Is there a simple fix for this? Right now I have both Systems.Collections and Systems.Collections.Generic at the top of the project - but they are now both greyed out. I'd really prefer to keep using List<> since I don't know what ArrayList is and I have another C# code that is passing in data via List<> already.