When adding service data to a Call service which has no data, the entire Choose section switches to YAML mode as soon as the first letter of the option name is typed:

Only the action should switch to YAML mode, looking similar to the mockup below:

choose_test:
alias: Choose Test
mode: single
sequence:
- choose:
- conditions:
- after: sunrise
before: sunset
condition: sun
sequence:
- entity_id: light.bedside_light
service: light.turn_on
default: []
brightness_pct: 20 to the service data for the light.turn_on call.The value of "data" is not supported by the UI editor, we support "Object" but received ""b"".You can still edit your config in yaml.
I get this in all actions, not limited to choose.
An example is a Call service action with light.turn_on.
As soon as a letter is entered in the Service data-field, the editor switches to yaml mode with the error message shown in OP.
Funny thing is that if the correct values are entered in yaml-mode, it is possible to switch back to UI-mode and everything is fine again and editable in UI.
Yes, it is similar in other actions but with those it is only the individual action which changes to YAML mode. With Choose, typing service data in any of of the nested actions within the Choose causes the whole Choose section to switch to YAML mode (not just the individual action).
For example, I have an automation with a Choose which has 4 options each with a condition and an action. Typing service data for any of those caused the whole Choose section to turn to YAML mode and not just the individual action.
The only workaround is to switch that action to YAML mode before entering the service data - you can manually switch an individual action to YAML mode, it's just that the error doesn't do this.
I have the same in 0.114.3 (in 0.113.3 it works OK)
looks like regresion bug after https://github.com/home-assistant/frontend/pull/6436
I get this error also, as already said the scripts aspect is irrelevant.
To replicate all I need to do is create a new automation in the UI, create an action to call a service, for example media_player.volume_set and then in the "Service data" section if I attempt to type volume_level: I get the error as soon as I type the first letter (ie. "v")
Service and entity selected, all that's left to do is type in volume_level: x into the circled Service Data section

This image then shows the error as soon as the first letter has been typed: 
I can get around this issue by pasting the data into the "Service Data" box (I'm assuming some validation is done and by doing it this way, all text is there instantly so no error can occur). This can also technically be "fixed" by formatting the data in the UI with correct YAML and then selecting the three dots in the corner and selecting "Edit with UI" so that the error doesn't persist.
I came here to make sure this issue has been reported, and also I can confirm a second work-around exists, for those who might prefer it:
If you're trying to type message: "My message here" into the service data box, you'll run into this issue the moment you type "m". However, if you instead type {message: "My message here"} it'll work just fine. If you need multiple items in that dictionary, you can just separate them via comma, {message: "My message here", title: "My title here"}
Apparently this issue isn't triggered if you use JSON instead of trying to type YAML.
And as soon as you save this JSON, it'll reformat it back to YAML, too.
I came here to make sure this issue has been reported, and also I can confirm a second work-around exists, for those who might prefer it:
If you're trying to type
message: "My message here"into the service data box, you'll run into this issue the moment you type "m". However, if you instead type{message: "My message here"}it'll work just fine. If you need multiple items in that dictionary, you can just separate them via comma,{message: "My message here", title: "My title here"}Apparently this issue isn't triggered if you use JSON instead of trying to type YAML.
And as soon as you save this JSON, it'll reformat it back to YAML, too.
This really helps, I don't leave the curly brackets in but instead I write {message: and then delete the { as message: is valid. Obviously not a fix but a very useful work around until it is fixed!
This is fixed in 0.115
Most helpful comment
This is fixed in 0.115