Using ha-entity-picker in Lovelace card configuration

I’ve built a GUI for a custom component, and wish to use this component (or something similar) to let the user change which entity it displays data for. However when I create one, I get the following error:

Uncaught (in promise) TypeError: this._comboBox is null

This is my code:

const entityPicker = document.createElement('ha-entity-picker');
            Object.assign(entityPicker, {
                label: "Entity",
                name: "entity",
                required: true,
                value: this.config.entity,
                onchange: this.updateSetting.bind(card)
            });
            outerDiv.append(entityPicker);

Having looked at the core code, I can’t see public fields with which to declare a combo box, I’d appreciate any pointers