Can there be obstruction if no underlying crime was committed?

Yes.  See the link below.

https://www.politifact.com/truth-o-meter/article/2019/mar/25/martha-stewart-donald-trump-can-there-be-obstructi/

Per Fact Checking sites.   They really are very good.  I can see why recently Trump attacked Fact Checking websites.  Many of the worst lies (fake news) are disseminated on Facebook by bloggers.  But on occasion,  statements by Donald Trump join the “Pants on Fire” group.  So, do not trust anything posted anonymously by a blogger.  Same goes for Trump’s tweets.   Secondly, there are a ton of commentators particularly on Fox News, who live by selling books and videos, on grand conspiracy theories and deep state nonsense.  I will not vote for anyone with a track record as bad as Trump’s in the 2020 election.

Learn to separate out serious news reporting from late night “news show.”    If you ever watched a Lou Dobbs broadcast, you know what I mean.  Lou makes it very easy for his guests to respond, as there’s hardly a question embedded inside of a question, except,  “Don’t you agree with me?”.

As per the Mueller report,  I support the Attorney General’s approach.  The amount of speculation of what is contained in the report, is really incredible.  His letter cited just two conclusions contained in the report.  Redacting sensitive information from a report, particularly one related to counter-intelligence, requires considerable time.  I reject the call from impatient Democrats, saying they want everything right now.  For our justice system to work, not everything should be disclosed, including FISA subpoenas.

Barr released his letter two days after he got the report because to do otherwise would have politicians claiming he was concealing information.   It is difficult to tamp down or narrow the scope of speculation on what the report concludes.

Stay tuned,

Dave