How to reproduce:
1. Create and open a new project
2. Set the 'External Script Editor' to 'TextEdit' (in the Preferences window)
3. Create a new C# script and open it
4. Observe the Console window

Expected result: no exceptions are thrown
Actual result: two exceptions are thrown

Reproducible with: 2019.2.5f1, 2019.2.11f1
Not reproducible with: 2017.4.33f1, 2018.4.12f1, 2019.2.0a1, 2019.2.4f1, 2019.3.0b9, 2020.1.0a11

Thrown exceptions:
1) MissingMethodException: Attempted to access a missing method.
Unity.CodeEditor.CodeEditor.OSOpenFile (System.String appPath, System.String arguments)
2) Exception thrown while invoking [OnOpenAssetAttribute] method ‘Unity.CodeEditor.CodeEditor:OnOpenAsset (int,int,int)’ : MissingMethodException: Attempted to access a missing method.
Unity.CodeEditor.CodeEditor.OSOpenFile (System.String appPath, System.String arguments)