And don’t forget to save your BitLocker recovery key somewhere outside encrypted partition. By installing Ubuntu, you end up with 2 ways to boot Windows: 1 is using its EFI loader directly, another is chainloading in Grub. The choice affects PCRs that are being checked during a boot process with BitLocker enabled, so if you always used one way, but decided to use another this time, you’ll be prompted to enter the recovery key. The only solution to re-configure the default is a) suspend BitLocker protection in its preferences (affects the next boot only IIRC); b) enter your key to boot the way you want it (via Grub / direct EFI loader) and willing to use further.