closes #51

[mpadge]

Just involved checking the input$layer setting on every operation. Should hopefully be good to go.

[Robinlovelace]

Thanks @mpadge will test now. Great work by the looks of it.

[Robinlovelace]

I get this error message @mpadge:

 runUpthat()
Loading required package: shiny

Listening on http://127.0.0.1:7112
Found these Rds files: ./net-accra.Rds, ./net-bristol.Rds, ./net-kathmandu.Rds, inst/net-ac1.Rds
Reading this matching file: ./net-accra.Rds
Warning: Error in if: argument is of length zero
  [No stack trace available]
Warning: Error in if: argument is of length zero
  [No stack trace available]

[Robinlovelace]

But also get that on the master branch...

[Robinlovelace]

The error seems to be due to something like this:

a = NULL
if(a == 7) {print("a")}
Error in if (a == 7) { : argument is of length zero

[Robinlovelace]

Putting on pause until I can debug the crashes on master... Any ideas @mpadge ?