Was Ellen White wrong about a temple in the New Jerusalem?

The answer to your question is actually quite clear biblically. Revelation 21 describes events that happen after the final destruction of sin (depicted in Revelation 20). When there is no more sin, there is no more need of a temple. But while sin remains, there is a temple with its ministry for dealing with sin. This is what Hebrews refers to in places such as Hebrews 8:1, 2 and 9:11ff.

Jesus is now our High Priest in the heavenly sanctuary (or temple), apply¬ing the benefits of His atoning sacrifice to our cases and wrapping up the final issues in the great controversy. In Early Writings, Mrs. White was shown the present situation—not the one that will exist after the destruction of sin. So her view of the heavenly sanctuary is in line with what the Bible teaches on this subject.