Spammed with a Null Reference exception in Animator

I don't know how to track down what asset is missing... this is an editor error, not an in-game issue. Works fine in-game. However, my game started taking 20-50 seconds to load after this error began: NullReferenceException: Object reference not set to an instance of an object UnityEditor.Graphs.AnimationStateMachine.TransitionEditionContext.BuildNames () (at C:/buildslave/unity/build/Editor/Graphs/UnityEditor.Graphs/AnimationStateMachine/TransitionEditionContext.cs:44) UnityEditor.Graphs.AnimationStateMachine.TransitionEditionContext..ctor (UnityEditor.Animations.AnimatorTransitionBase aTransition, UnityEditor.Animations.AnimatorState aSourceState, UnityEditor.Animations.AnimatorStateMachine aSourceStateMachine, UnityEditor.Animations.AnimatorStateMachine aOwnerStateMachine) (at C:/buildslave/unity/build/Editor/Graphs/UnityEditor.Graphs/AnimationStateMachine/TransitionEditionContext.cs:28) UnityEditor.Graphs.AnimationStateMachine.AnimatorTransitionInspectorBase.ComputeTransitionContexts () (at C:/buildslave/unity/build/Editor/Graphs/UnityEditor.Graphs/AnimationStateMachine/AnimatorTransitionInspectorBase.cs:125) UnityEditor.Graphs.AnimationStateMachine.AnimatorTransitionInspectorBase.OnEnable () (at C:/buildslave/unity/build/Editor/Graphs/UnityEditor.Graphs/AnimationStateMachine/AnimatorTransitionInspectorBase.cs:87) UnityEditor.Graphs.AnimationStateMachine.AnimatorStateTransitionInspector.OnEnable () (at C:/buildslave/unity/build/Editor/Graphs/UnityEditor.Graphs/AnimationStateMachine/AnimatorStateTransitionInspector.cs:55)