If you click on "Set budgeted amount" leaving the amount blank, it gives a fatal error.

"SQLSTATE[23000]: Integrity constraint violation: 19 NOT NULL constraint failed: budget_limits.amount (SQL: insert into "budget_limits" ("budget_id", "transaction_currency_id", "start_date", "end_date", "amount", "updated_at", "created_at") values (3, 7, 2020-06-01 00:00:00, 2020-06-30 00:00:00, ?, 2020-06-03 19:10:59, 2020-06-03 19:10:59))"

Looks like there's no validation for that field.