Issue with structure_plot when subsampling and all samples from a group are removed

[eweine]

There appears to be an issue in structure_plot when the n argument is less than the total number of rows of L, and some of the groups that are supplied in the grouping argument have a very small number of samples.

The issue typically occurs here: https://github.com/stephenslab/fastTopics/blob/master/R/structure_plot.R#L256

On line 256, if the length of i is 0, the variable y will not have been created, and this will result in the error

Error in structure_plot(...) : 
  object 'y' not found

I think there are two ways to fix this. Either a) do some sort of "smart" sampling, where you make sure to at least sample 1 row from each group or b) keep the sampling the same but somehow work around this issue.

[pcarbo]

@eweine I ran a small test in which some of the groups disappeared after subsampling, and I was unable to generate an error.
Do you have a small example I could use to reproduce the error? I wonder if perhaps the error only occurs when some combination of the settings are used.