Resolved Animator Controller invalid when turning into a prefab.

Every time I change this to a prefab it says that f******* message in yellow. But it happens to play animation as normal. WTF is going on?

When I click on the controller, THAT ACTUALLY EXIST, it shows this other error in red:

ullReferenceException: Object reference not set to an instance of an object
UnityEditor.Graphs.AnimationStateMachine.Graph.GenerateConnectionKey (UnityEditor.Graphs.Node srcNode, UnityEditor.Graphs.Node dstNode) (at <00578d709e2b43b38413b2b2b34f6fbe>:0)
UnityEditor.Graphs.AnimationStateMachine.Graph.GetEdgeInfo (UnityEditor.Graphs.Edge edge) (at <00578d709e2b43b38413b2b2b34f6fbe>:0)
UnityEditor.Graphs.AnimationStateMachine.EdgeGUI.DoEdges () (at <00578d709e2b43b38413b2b2b34f6fbe>:0)
UnityEditor.Graphs.AnimationStateMachine.GraphGUI.OnGraphGUI () (at <00578d709e2b43b38413b2b2b34f6fbe>:0)
UnityEditor.Graphs.AnimatorControllerTool.StateMachineView (UnityEngine.Rect position, System.Single zoomLevel) (at <00578d709e2b43b38413b2b2b34f6fbe>:0)
UnityEditor.Graphs.AnimatorControllerTool.DoGraph (UnityEngine.Rect graphRect, System.Single zoomLevel) (at <00578d709e2b43b38413b2b2b34f6fbe>:0)
UnityEditor.Graphs.AnimatorControllerTool.<SetupGUI>b__143_12 () (at <00578d709e2b43b38413b2b2b34f6fbe>:0)
UnityEditor.Graphs.AnimatorControllerTool.ScopedOnGUI (System.Action onGUIHandler) (at <00578d709e2b43b38413b2b2b34f6fbe>:0)
UnityEditor.Graphs.AnimatorControllerTool.<SetupGUI>b__143_10 () (at <00578d709e2b43b38413b2b2b34f6fbe>:0)
UnityEngine.UIElements.IMGUIContainer.DoOnGUI (UnityEngine.Event evt, UnityEngine.Matrix4x4 parentTransform, UnityEngine.Rect clippingRect, System.Boolean isComputingLayout, UnityEngine.Rect layoutSize, System.Action onGUIHandler, System.Boolean canAffectFocus) (at <8759e14def7c40b68c657c638e8a0434>:0)
UnityEngine.UIElements.IMGUIContainer.HandleIMGUIEvent (UnityEngine.Event e, UnityEngine.Matrix4x4 worldTransform, UnityEngine.Rect clippingRect, System.Action onGUIHandler, System.Boolean canAffectFocus) (at <8759e14def7c40b68c657c638e8a0434>:0)
UnityEngine.UIElements.IMGUIContainer.DoIMGUIRepaint () (at <8759e14def7c40b68c657c638e8a0434>:0)
UnityEngine.UIElements.UIR.RenderChainCommand.ExecuteNonDrawMesh (UnityEngine.UIElements.UIR.DrawParams drawParams, System.Single pixelsPerPoint, System.Exception& immediateException) (at <8759e14def7c40b68c657c638e8a0434>:0)
Rethrow as ImmediateModeException
UnityEngine.UIElements.UIR.RenderChain.Render () (at <8759e14def7c40b68c657c638e8a0434>:0)
UnityEngine.UIElements.UIRRepaintUpdater.Update () (at <8759e14def7c40b68c657c638e8a0434>:0)
UnityEngine.UIElements.VisualTreeUpdater.UpdateVisualTreePhase (UnityEngine.UIElements.VisualTreeUpdatePhase phase) (at <8759e14def7c40b68c657c638e8a0434>:0)
UnityEngine.UIElements.Panel.UpdateForRepaint () (at <8759e14def7c40b68c657c638e8a0434>:0)
UnityEngine.UIElements.Panel.Repaint (UnityEngine.Event e) (at <8759e14def7c40b68c657c638e8a0434>:0)
UnityEngine.UIElements.UIElementsUtility.DoDispatch (UnityEngine.UIElements.BaseVisualElementPanel panel) (at <8759e14def7c40b68c657c638e8a0434>:0)
UnityEngine.UIElements.UIElementsUtility.UnityEngine.UIElements.IUIElementsUtility.ProcessEvent (System.Int32 instanceID, System.IntPtr nativeEventPtr, System.Boolean& eventHandled) (at <8759e14def7c40b68c657c638e8a0434>:0)
UnityEngine.UIElements.UIEventRegistration.ProcessEvent (System.Int32 instanceID, System.IntPtr nativeEventPtr) (at <8759e14def7c40b68c657c638e8a0434>:0)
UnityEngine.UIElements.UIEventRegistration+<>c.<.cctor>b__1_2 (System.Int32 i, System.IntPtr ptr) (at <8759e14def7c40b68c657c638e8a0434>:0)
UnityEngine.GUIUtility.ProcessEvent (System.Int32 instanceID, System.IntPtr nativeEventPtr, System.Boolean& result) (at <44335255efcb428fb633c913db3eed7a>:0)

RESOLUTION: Reinstall Unity