Custom Search Filter in List page

Hello Community, 

I have a requirement to design a custom search filter in list page in Freedom UI, so that I have a text field and a button.

Now, on entering a number (Plate Number) in the field and on click of button, I wanted to filter only the records that contain the plate number. 

The filter works perfectly. But now when I navigate to any other section, I am seeing a popup message (Un save popup) as shown below.

To clear this error, I tried to save the number entered in the field and the issue is gone until I search for other number. 

Now I received a system error as shown below. 

How can I resolve this?

Any suggestions are really helpful.