How to reproduce:
1. Create and open a project
2. In 'Preferences' > 'External Tools' set Visual Studio Code as your preferred External Script Editor
3. Re-open the project
4. Observe the Console window

Expected result: no errors are thrown
Actual result: IndexOutOfRangeException is thrown

Reproducible with: 2019.2.20f1, 2019.3.0f6, 2020.1.0a22 (1.1.4)
Not reproducible with: 2019.2.20f1, 2019.3.0f6, 2020.1.0a22 (1.0.2, 1.1.2, 1.1.3)
Could not test with: 2017.4.36f1, 2018.4.17f1 (package not available)

Note: in 2019.2.20f1 the issue reproduces after installing the 1.1.4 package version instead of opening the Editor

Thrown exception:
IndexOutOfRangeException: Index was outside the bounds of the array.
VSCodeEditor.ProjectGeneration.<SyncIfNeeded>b__25_0 (System.String asset) (at Library/PackageCache/com.unity.ide.vscode@1.1.4/Editor/ProjectGeneration/ProjectGeneration.cs:175)
System.Linq.Enumerable+SelectListIterator`2[TSource,TResult].MoveNext () (at <351e49e2a5bf4fd6beabb458ce2255f3>:0)
System.Linq.Enumerable+ConcatIterator`1[TSource].MoveNext () (at <351e49e2a5bf4fd6beabb458ce2255f3>:0)