This error is recoverable, but I would still rank it pretty high as it leads to a disuptive user experience.

Steps to reproduce:

1. In the browser open up the developer tools
2. Create a new dashboard and add a 'Graph' widget
3. Select a host and add any metric
4. Clone the dataset.
5. Delete only the first dataset completely by selecting the 'x' in the top right of the widget configuration
6. In the remaining dataset, start trying to type in the Item pattern' field and notice that multiselect stops working. In the browser developer tools notice an error.
7. The only way to recover from this is to add another dataset.
8. The same behavior will occur if you have 3 datasets as well. In this case, you create 3 datasets, delete either the first or second one. Then in the last dataset attempt to start typing in the Item pattern field and notice the error. The host pattern field continues to work fine