The method `DefaultModuleService#getSingleItem` is used for both inputs and outputs, but this line only checks the input: https://github.com/scijava/scijava-common/blob/beb6de2d8a31d3e783123a25eefc8b7e9dbef855/src/main/java/org/scijava/module/DefaultModuleService.java#L498 See also https://github.com/scijava/scijava-common/pull/423#discussion_r633126331.
The method
DefaultModuleService#getSingleItemis used for both inputs and outputs, but this line only checks the input:scijava-common/src/main/java/org/scijava/module/DefaultModuleService.java
Line 498 in beb6de2
See also #423 (comment).