-e: see title, feature looks working otherwise
--no such exception thrown in 5.4.0b22

-repro:
--open attached project or create new empty one
--build empty scene for webgl
--open the build report url
--go to stripping
--click calculate size
--NOTICE an exception is thrown in the editor:
---KeyNotFoundException: The given key was not present in the dictionary.
System.Collections.Generic.Dictionary`2[System.String,System.Int32].get_Item (System.String key) (at /Users/builduser/buildslave/mono/build/mcs/class/corlib/System.Collections.Generic/Dictionary.cs:150)
UnityEditor.WebGL.WebGLStrippingInfo.Analyze () (at /Users/builduser/buildslave/unity/build/PlatformDependent/WebGL/Extensions/Unity.WebGL.extensions/AnalyzeOutputSize.cs:276)