Internal Unity Error when trying to save a Prefab.

This happens every time I try to save the Prefab:

 NullReferenceException: Object reference not set to an instance of an object
 UnityEditor.PrefabUtility.RegisterNewObjects (System.Collections.Generic.List`1 newHierarchy, System.Collections.Generic.List`1 hierarchy, System.String actionName) (at /Users/builduser/buildslave/unity/build/Editor/Mono/ImportSettings/PrefabUtility.cs:121)
 UnityEditor.PrefabUtility.ReplacePrefabWithUndo (UnityEngine.GameObject target) (at /Users/builduser/buildslave/unity/build/Editor/Mono/ImportSettings/PrefabUtility.cs:223)
 UnityEditor.GameObjectInspector.DoPrefabButtons (PrefabType prefabType, UnityEngine.GameObject go) (at /Users/builduser/buildslave/unity/build/Editor/Mono/Inspector/GameObjectInspector.cs:347)
 UnityEditor.GameObjectInspector.DrawInspector () (at /Users/builduser/buildslave/unity/build/Editor/Mono/Inspector/GameObjectInspector.cs:260)
 UnityEditor.GameObjectInspector.OnHeaderGUI () (at /Users/builduser/buildslave/unity/build/Editor/Mono/Inspector/GameObjectInspector.cs:170)
 UnityEditor.Editor.DrawHeader () (at /Users/builduser/buildslave/unity/build/Editor/Mono/Inspector/Editor.cs:420)
 UnityEditor.InspectorWindow.DrawEditor (UnityEditor.Editor[] editors, Int32 editorIndex, Boolean rebuildOptimizedGUIBlock, System.Boolean& showImportedObjectBarNext, UnityEngine.Rect& importedObjectBarRect) (at /Users/builduser/buildslave/unity/build/Editor/Mono/Inspector/InspectorWindow.cs:1117)
 UnityEditor.InspectorWindow.DrawEditors (UnityEditor.Editor[] editors) (at /Users/builduser/buildslave/unity/build/Editor/Mono/Inspector/InspectorWindow.cs:1022)
 UnityEditor.InspectorWindow.OnGUI () (at /Users/builduser/buildslave/unity/build/Editor/Mono/Inspector/InspectorWindow.cs:361)
 System.Reflection.MonoMethod.Invoke (System.Object obj, BindingFlags invokeAttr, System.Reflection.Binder binder, System.Object[] parameters, System.Globalization.CultureInfo culture) (at /Users/builduser/buildslave/mono/build/mcs/class/corlib/System.Reflection/MonoMethod.cs:222)
 Rethrow as TargetInvocationException: Exception has been thrown by the target of an invocation.
 System.Reflection.MonoMethod.Invoke (System.Object obj, BindingFlags invokeAttr, System.Reflection.Binder binder, System.Object[] parameters, System.Globalization.CultureInfo culture) (at /Users/builduser/buildslave/mono/build/mcs/class/corlib/System.Reflection/MonoMethod.cs:232)
 System.Reflection.MethodBase.Invoke (System.Object obj, System.Object[] parameters) (at /Users/builduser/buildslave/mono/build/mcs/class/corlib/System.Reflection/MethodBase.cs:115)
 UnityEditor.HostView.Invoke (System.String methodName, System.Object obj) (at /Users/builduser/buildslave/unity/build/Editor/Mono/HostView.cs:295)
 UnityEditor.HostView.Invoke (System.String methodName) (at /Users/builduser/buildslave/unity/build/Editor/Mono/HostView.cs:288)
 UnityEditor.HostView.InvokeOnGUI (Rect onGUIPosition) (at /Users/builduser/buildslave/unity/build/Editor/Mono/HostView.cs:255)
   

Even if I delete the Prefab file and try to make a new one, I get the same result. How the heck do I fix this? Unity version 2017.3.0f3