How to reproduce:
1. Open the attached "1065491_repro.zip" project
2. Open "Prefabs/Parent" prefab
3. Copy "Nested" prefab from the Hierarchy
4. Open "Prefabs/Child_2" prefab
5. Paste the copied prefab to the Hierarchy
-- Editor crash

Reproducible with - 2018.2.0x-ImprovedPrefabs (ff19e2e229d2), 2018.3.0a7

Note: The issue is only reproducible when copying GameObject that is nested inside sub-prefab in the prefab Hierarchy, and that actually belongs to the sub-prefab (it is present in the Hierarchy when opening the sub-prefab).

Stack trace:
(Unity) PasteGameObjectPasteboardData
(Unity) PasteGameObjectsFromPasteboard
(Unity) Unsupported_CUSTOM_PasteGameObjectsFromPasteboard
(Mono JIT Code) (wrapper managed-to-native) UnityEditor.Unsupported:PasteGameObjectsFromPasteboard ()
(Mono JIT Code) [C:\buildslave\unity\build\Editor\Mono\SceneHierarchy.cs:1230] UnityEditor.SceneHierarchy:PasteGO ()
(Mono JIT Code) [C:\buildslave\unity\build\Editor\Mono\GUI\GenericMenu.cs:121] UnityEditor.GenericMenu:CatchMenu (object,string[],int)
(Mono JIT Code) (wrapper runtime-invoke) <Module>:runtime_invoke_void__this___object_object_int (object,intptr,intptr,intptr)
(mono) [c:\buildslave\mono\build\mono\mini\mini.c:4937] mono_jit_runtime_invoke
(mono) [c:\buildslave\mono\build\mono\metadata\object.c:2623] mono_runtime_invoke
(Unity) scripting_method_invoke
(Unity) ScriptingInvocation::Invoke
(Unity) CustomPopupMenuInterface::Execute
(Unity) MenuController::ExecuteMenuItem
(Unity) ShowDelayedContextMenu
(Unity) GUIView::OnInputEvent
(Unity) GUIView::ProcessInputEvent
(Unity) GUIView::DoPaint
(Unity) GUIView::RepaintAll
(Unity) Application::TickTimer
(Unity) MainMessageLoop
(Unity) WinMain
(Unity) __scrt_common_main_seh
(KERNEL32) BaseThreadInitThunk
(ntdll) RtlUserThreadStart

Fixed in 2019.1.0a5
Backported to 2018.3.0b7