Insert value in a culumn of "integer references"

I have a table in SQLite that is linked to a main table using: integer references. When I add rows to that table, should I give it a value, or should I just keep it null?

Here is my table:

CREATE TABLE table1(
  id integer PRIMARY KEY,
  firstCollumn text,
);

CREATE TABLE table2(
  id integer PRIMARY KEY,
  firstColumn text,
  tableID integer references table1(id)
);

When I gave it a null value, and I later tried retrieving it, it gave me problems.

If I'm not supposed to give it a null, then how do I know what value to give it?