How to reproduce:
1. Open attached project "case1259902.zip"
2. Select CubeTimeline.playable asset in Project window
3. Open Timeline window
4. Select "Recorded" clip in Timeline
5. Click to edit "In" Blend Curve
6. Create two keys by double-clicking on the curve
7. Click on one of the curve presets at the bottom of the Curve Editor
8. Create a key by double-clicking on the curve

Expected result: No error appears
Actual result: Error "IndexOutOfRangeException: GetKey" appears

Reproducible with: 2019.4.2f1, 2020.1.0b15, 2020.2.0a16
Cannot test with: 2018.4.23f1 (errors when downgrading the project)

Reproducible with package versions: 1.2.6, 1.5.0-preview.2
Cannot test with package versions: 1.2.0, 1.2.5 (errors when downgrading the project to lower version than 2019.4.0f1)