upd. Regression was checked on //UE4/Release-4.26 CL 15973114, issue was reproduced. Regression - No
If you click cancel, nothing is added except a Timeline reference component under Variables (even though no Timeline node is actually added). The resulting component can't be deleted from My Blueprints (a normal timeline can't either, you delete those by selecting the node, but without the node in the graph to select you're stranded)
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-126467 in the post.