Exactly as the title says. If you set MultipleSelectionSettings.Enabled="True" and LayoutSettings.TextBoxMode="ReadOnly" it is not possible to add rows on the client side. See the example.

Steps to reproduce:

1. Run sample.
2. Activate combo dropdown, rows will load fine with checkboxes to the left.
3. Add a new row using the button below the combo. Alert confirms add.
4. Try to activate the combo dropdown again.
5. Error.

You can try the same thing but remove either of the lines below from the combo declaration and you wont recieve an error.

<MultipleSelectionSettings Enabled="true" />
<LayoutSettings TextBoxMode="ReadOnly"></LayoutSettings>

We could really use a work around or hotfix. Thanks.