This is (what I would consider) a glitch with SQLite. This error occurs whether there are any records in the table or not.
When adding a table from scratch, you can specify NOT NULL. However, you can’t do this when adding a column. SQLite’s specification says you have to have a default for this, which is a poor choice.
You could specify the default value I guess, such as
Schema::table('organisations', function (Blueprint $table){
$table->string('long_name')->default('default_value');
});