Estimates: New estimate: "Amount" is not marked as obligatory.

[Clairebaer]

Describe the bug
New estimate: "Amount" is not marked as obligatory. When trying to save an error occurs:
SQLSTATE[23000]: Integrity constraint violation: 1048 Column 'amount' cannot be null

To Reproduce
Steps to reproduce the behavior:

1. Go to Estimates
2. Click on "New estimate"
3. Fill out project and title, leave "Amount" empty
4. Click on "Save"
5. See error

Expected behavior
"Amount" is marked as obligatory. Saving without an amount is not possible (a notice of missing information is shown).

Screenshots
If applicable, add screenshots to help explain your problem.

System (please complete the following information):

• Sales version: 2.1.9