How to reproduce:
1. Open attached project "HttpWebRequestLeakTest" and enter Play Mode
2. Open Memory Profiler
3. Close Memory Profiler and exit Play Mode
----------------------------
Observe console

Error:
NullReferenceException: Object reference not set to an instance of an object
Unity.MemoryProfiler.Editor.MemoryProfilerWindow.PlaymodeStateChanged (UnityEditor.PlayModeStateChange playModeState) (at Library/PackageCache/com.unity.memoryprofiler@0.1.0-preview.5/Editor/MemoryProfilerWindow.cs:1024)
UnityEditor.EditorApplication.Internal_PlayModeStateChanged (UnityEditor.PlayModeStateChange state) (at C:/buildslave/unity/build/Editor/Mono/EditorApplication.cs:302)

Reproduced with: Memory Profiler Package 0.1.0-preview.5, Unity 2018.3.8f1, 2019.1.0b6, 2019.2.0a8

Note: Memory Profiler was introduced in 2018.3.0f2