For any curious why this deal has to wait until July 1st until it can be formally done, there are two separate reasons:
1) Group 2 free agents (RFA) like Nichushkin have to be signed by Dec 1st or they are ineligible to sign/play that season. So he could not be signed for 2017-18 after Dec 1st.
2) Generally players are not permitted to sign contracts that do not include the current season (League Year). This is why players like Kovalchuk and Koskinen have to wait until July 1st as well, even though they are no longer under contract to any KHL team. There are three exceptions to this rule**, however Nichushkin does not fit into any of those exception categories.
** The exceptions are:
a) Contract extension. Nichushkin isn't currently under NHL contract so he's not eligible to sign a contract extension.
b) Unsigned draft pick. They can be signed from March 1st to June 1st for the upcoming season.
c) Draft Related Free agent. They can be signed from March 1st to June 1st for the upcoming season. Generally draft related free agents are those players who are eligible to sign with any team as a UFA and have never been signed to an NHL contract before. e.g. Shipachyov last season.