drag the progress bar in level sequence would crash in debug editor

it seems in FSequencerTimeSliderController::ComputeFrameTimeFromMouse
the CursorPos is -nan

use debug editor in vs to run the editor
1. create a level sequence asset
2. save asset (must save)
3. drag the mouse point from green point to red point (in picture)
4. crash in checkSlow(InSubFrame >= 0.f && InSubFrame < 1.f);