Periodic Table Database - Build a Periodic Table Database

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I am unable to set the type_id column as NOT NULL, i tried everything i know. I tried deleting it and adding it again setting it as INT NOT NULL but it didint work, so i just created the column as INT, then updating the column with the NOT NULL data type. Every time i try, terminal says it contains null values however i havent added any values yet into any columns. I tried setting every column as not null of every table but it didnt work. I dont know what else i can do. Ill leave a link to my git repository where i have written down every bash command along with the comment/task for what the command was oriented.

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Periodic Table Database - Build a Periodic Table Database

I solved it by setting a default value of 1 after setting it as not null, line of command was "ALTER TABLE properties ADD COLUMN type_id INT NOT NULL DEFAULT 1 REFERENCES types(type_id);