We have a bit of code on our system which is failing at times, and when it does results in some ugly error code on the final webpage.
The Freemarker code in question is trying to return back to Squiz Matrix some markup code which is expecting a particular value ${s.result…} value to be populated, but without any default value (as per https://docs.funnelback.com/more/extra/freemarker.html#data-model-variables) or any attempt at exception handling where the ${s.results…} value is blank.
Going forward though, it would be best to prevent the exception from happening in the first place by always being defensive when accessing any variable. You can do this by checking it if exists before using it. This is what I commonly do wit ``hin my templates:
<#if (s.result.summary)!?has_content>
<#-- you code -->
</#if>
The above makes use of the ! and ?has_content functions to test if a parameter exists and is not empty.