HTTP Callout action in flow returns internal server error 502

Question

I am first time trying out the HTTP callout action inside of the flow. I have added a query string parameter called content and ran the flow using the debug button. I see the expected response but along with that I see an Internal server error message as well. If i call the same api using Postman it get a 200 response. Is there anything I am doing wrong with the setting of the HTTP callout action? Here is the snip -

I am adding here what query string paramter I am sending.

And here is what I see on AWS Cloudwatch. I see the response of the content query parameter and why does it say null as in the snip below?

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EDIT - After sfdcfox answer -

The API that I had called did have some issues which I was able to rectify.

I am still not able to make a successful callout from the flow so far. This is error I am seeing when I run the flow. I do see from the error that it is something wrong that I have done on the mapping.

Here is what I see after a successful run of my function in Lambda -

This is how I set my Query Parameter keys-

And this is how I set my response mapping. Maybe a dumb question. why it is I am getting my response in the body property when I pass in content as my query parameter string or am I missing something here.

EDIT - After sfdcfox editted answer -

THanks @sfdcfox for the continued help. The below is what I have for the Query Parameter Keys-

And this is what I have as Sample Request -

This is the error now and I am not sure where the body2 comes from -

Answer 1

Without knowing what API you're calling or anything, I can see your problem fairly immediately. You're setting the query string to endpoint?content=response+from+the+api. However, this endpoint is parsing JSON, presumably from the body. You haven't generated any JSON, so the data is null, and when it tries to read the property of null, you get a null pointer exception. The server language is, of course, a flavor of JavaScript, which is why you're specifically getting Cannot read properties of null (reading 'content'). You need a JSON payload in order to submit the parameters.

See the documentation, specifically:

For POST, PUT, PATCH, and DELETE, provide a sample API request body. Salesforce generates a data structure from the sample request.

• Click New.
• Paste a sample JSON request.
• Click Review and confirm that the Apex data structure is correct.
• To make changes, edit the text in Sample JSON Request or select data types from the data structure.
• Click Review.
• Click Done.

You need to click on the button circled below to open the appropriate dialog: