Fix calling methods on plot element (#323)
Fix calling methods on plot element (#323)

A DatasetActivity may not have a dataset if the data cannot be
plotted (for example, if the dataset is 0- or 3-dimensional).

Review: da39a3ee5e6b4b0d3255bfef95601890afd80709@kunalq 
labrad/scalabrad-web,labrad/scalabrad-web,labrad/scalabrad-web,labrad/scalabrad-web,labrad/scalabrad-web