Reproduction steps:
1. Open the attached project.
2. Build and run on WebGL.
3. Click "LoadFromMemory-OK" button.
4. Open a web console.
- Notice, that no errors are present.
5. Click "LoadFromMemory-NG" button.
6. Open a web console.

Actual behavior:
- Notice the error message:
---
Failed to decompress data for the AssetBundle 'Memory'.
UnityEngine.AssetBundle:LoadFromMemory(Byte[], UInt32)
UnityEngine.AssetBundle:LoadFromMemory(Byte[])
<LoadAsserBundle>c__Iterator0:MoveNext()
UnityEngine.SetupCoroutine:InvokeMoveNext(IEnumerator, IntPtr)

[ line 437]
(Filename: Line: 437)

NullReferenceException: A null value was found where an object instance was required.
at ABLoadTest+<LoadAsserBundle>c__Iterator0.MoveNext () [0x00000] in <filename unknown>:0
at UnityEngine.SetupCoroutine.InvokeMoveNext (IEnumerator enumerator, IntPtr returnValueAddress) [0x00000] in <filename unknown>:0
---

Reproduced with:
2017.3.0a3, 2017.4.2f2, 2018.1.0f2, 2018.2.0b3, 2018.3.0a1

Not reproducible with:
2017.3.0a2

Tested with:
WebGL
MacOS 10.13.4
Mozilla Firefox 59.0.2