The null reference exception is of course not expected so it's not an error we can report more info on, it shouldn't happen at all.
It's likely caused by a unique constraint on a table T which has a field which isn't in the table (based on name).
I don't know how many unique constraints there are in your schema, the designer uses the following query:
SELECT ALL_CONSTRAINTS.*, ALL_CONS_COLUMNS.COLUMN_NAME, ALL_CONS_COLUMNS.POSITION
FROM ALL_CONSTRAINTS, ALL_CONS_COLUMNS WHERE ALL_CONSTRAINTS.CONSTRAINT_NAME = ALL_CONS_COLUMNS.CONSTRAINT_NAME
AND ALL_CONSTRAINTS.OWNER = ALL_CONS_COLUMNS.OWNER AND ALL_CONSTRAINTS.CONSTRAINT_TYPE='U'
AND ALL_CONSTRAINTS.OWNER IN ('schema1', 'schema2', ...)
ORDER BY ALL_CONSTRAINTS.OWNER ASC, ALL_CONSTRAINTS.TABLE_NAME ASC, ALL_CONSTRAINTS.CONSTRAINT_NAME ASC, ALL_CONS_COLUMNS.POSITION ASC
please adjust the
('schema1', 'schema2', ...) section with the schema name(s) of your project, if you use one schema you can just fill in one schema there.
It should give a resultset of all unique constraints of all tables in the schema and the fields in the unique constraints. As we trim the spaces off the names in the unique constraint data returned by that query, my suspicion is that the name of the field in the constraint after that doesn't match with the field in the table.