Check and make sure the Virtual Gamepad Emulation Bus is listed in the Device Manager/System Devices. If not you didn't install the ViGEm drivers correctly. If it's listed I would try re-downloading the Nightly Build. Delete the old plugin folder and replace it with the new download. I've had a corrupted Plugin folder before which lead to an empty dropdown. And make sure you reboot to load the drivers into the system.
Asus STRIX X299-E GAMING MB, i7-7820X 3.6/4.5GHz 8-Core, 32GB Corsair Dominator Platinum (CMD16GX4M2B2400C10), NVIDIA TITAN X (Pascal), Samsung 970 Pro 1TB M.2-NVMe w/Western Digital 2TB (WD2000F9YZ), Samsung 65Q9FN QLED, HTC Vive, 850w PS, W10