Graph widget throws errors when Map created with image elements is chosen as data source for Override host field.

Steps to reproduce:
1. Create Map with element type image
2. Create dashboard or use existing one
3. Add Map widget from step 1
4. Add Graph widget, for Override host field select -> widget and choose Map widget (from step 3)

Result:
Error appears:
Failed to update widget properties.

Further actions with Graph widget entails other errors:

• Action 1: By clicking Cancel button on Graph widget new error appears: Error: Unsupported state change. at... Widget is closed, but header controls are disabled, except for the Cancel button. Changes aren't saved.

• Action 2: By clicking Add button on Graph widget, widget doesn't appear on dashboard, but the space where widget should be displayed isn't clickable anymore. Graph widget appears after clicking Save changes on header.

• Action 3: Try to delete Graph widget. Error appears: TypeError: can't access property "selementid", closest_element is null at... Graph widget isn't deleted after clicking Save changes on header.

• Action 4: Try to edit Graph widget. Edit widget window is opened, error is displayed: Invalid parameter "Data set/1/hosts": cannot be empty. Delete value from Override host field, and click Apply, Save changes on dashboard. Changes not saved, value isn't deleted.

Expected:
No errors appear and further actions with widget are successful.