No, it wasn't. The straw purchase was, and Rittenhouse's friend got in trouble for making it. That, in turn, lead to him testifying against Rittenhouse as part of a plea agreement. Rittenhouse didn't actually take possession of the gun until he was 18 and did not face any charges related to the purchase of the gun, which he had destroyed after trial to avoid it becoming a symbol.

There was also a separate charge against Rittenhouse for carrying it that was dismissed at the very end of the trial due to a confusingly-worded law. But Rittenhouse wasn't carrying anything that counted as a short-barreled rifle. The judge ruled that the law did not apply to what Rittenhouse did, since the prosecutor did not allege any facts that the jury could have agreed with that would have made him guilty. The prosecutor was apparently going by the caption of the law (which says something about guns held by someone under 18) but that has no legal effect and the judge admonished him for trying to pull a fast one again.

This was covered right before it went to the jury as I recall, if you want to look up videos of how it went down at trial.

The gun laws were written incredibly poorly, listing at one point "all dangerous weapons" then for minors, citing an anti short barreled shotgun and rifle law.

The defense successfully argued that the phrasing meant rifles and shotguns with a barrel length of sixteen inches or more were exempted.