Since you haven't free motion quilted before, I think I would start quilting in the white areas with white thread. Start in the center of the sandwich right at the tree stem (not on the colour fabric) and stitch the straight lines between the blocks to the edge with a walking foot if you have one. Turn the quilt and stitch out with white on the opposite side. Continue in this manner until all the white SITD lines coming out from the tree are done. This will stabilize the quilt and depending on the batting you might not much more quilting in the background. You can then decide how to quilt the tree part. Beautiful Twister!