Creating connections and executing queries/updates
You've written a wrapper around JDBC that automatically connects/disconnects, and swallows exceptions.  Don't do that.

JDBC wants you to establish connections explicitly because it's a high-overhead activity.  Connecting involves a TCP handshake, authentication, opening the requested database, and some work to set up a session (which includes some locale settings, for example).  You have just taken a good design decision of JDBC (and all other database APIs) and made it worse.

Furthermore, if every query/update is in its own connection, then you can't execute multiple statements in a transaction.  (MySQL users often don't appreciate the value of transactions.)

Finally, SQLException exists for a reason: they allow your code to gracefully handle failures.  You merely swallow the exceptions after printing a stack trace.  You might as well be writing your data to /dev/null if you don't care whether your writes succeeded.  Also, returning null instead of propagating an exception does not make your caller's code any simpler.

I strongly recommend discarding your entire MySQLManager class and using JDBC as is.