There are a number of codecs that don’t work well within FCP for various reasons (most of the MPEG codecs, the Avid codec, None).
“None” was never intended to be an editing codec. Its purpose is to move media losslessly between different programs and platforms. Its very processor intensive to play this type of file…though Quicktime player can handle it on a robust system, QT Player uses far fewer system resources than FCP, which is doing many other things than just playing back the video.
You did the right thing converting it one of the uncompressed editing codecs.