DatabaseItem constructor Null safety
Implementation
DatabaseItem(this.id, {required Map<String, String> schema})
: assert(id != '', 'Id cannot be an empty string'),
assert(schema.isNotEmpty, 'Database item schema must be supplied'),
// Set db columns to an unmodifiable map with upper case keys
_itemSchema = Map.unmodifiable(
schema.map((key, value) => MapEntry(key, value.toUpperCase())));