A race can happen between oplog applier thread and index build coordinator thread and leading to this invariant failure. Oplog applier on receiving abortIndexBuild oplog entry performs the below steps.

1) Oplog applier thread signals abort to index builder.
2) Then calls the IndexBuildsCoordinator::_getIndexBuild() to get the ReplIndexBuildState for that index build.
3) Then waits for the index build to complete by waiting on the ReplIndexBuildState:: sharedPromise

Between step 1 and step 2, the index builder might have aborted and torn down the index build. In such cases, the step 2 would return "NoSuchKey" error.