To reproduce:

1. Download attached project “project_610124.zip" and open in Unity
2. Open “scene” scene
3. Enter to Play mode
4. Observe that ArgumentNullException is thrown in Console:

"ArgumentNullException: Argument cannot be null.
Parameter name: _unity_self
UnityEngine.WWW.get_keepWaiting () (at /Users/builduser/buildslave/unity/build/Modules/UnityWebRequestWWW/Public/WWW.cs:316)
UnityEngine.CustomYieldInstruction.MoveNext () (at /Users/builduser/buildslave/unity/build/Runtime/Export/CustomYieldInstruction.cs:19)
UnityEngine.SetupCoroutine.InvokeMoveNext (IEnumerator enumerator, IntPtr returnValueAddress) (at /Users/builduser/buildslave/unity/build/Runtime/Export/Coroutines.cs:17)"

Notes:
- This issue appears on both Windows and macOS
- It seems that this issue was fixed in Unity 2017.2: https://issuetracker.unity3d.com/issues/nullreferenceexception-is-thrown-when-the-dispose-method-is-inside-the-using-block

Reproduced on Unity 2017.4.0f1, 2017.4.17f1, 2018.2.20f1, 2018.3.3f1, 2019.1.0a14 and 2019.2.0a1