hbox(p1,p2) emits StackOverflowError

is it not possible to create an interactive graphic with two plots? one button and one plot works, but not two plots.

```
julia> using Interact, Mux, PlotlyJS, WebIO

julia> b = button()

julia> p1 = plot(rand(10));

julia> p2 = plot(rand(10));

julia> ui = hbox(p1,b); ### this works

julia> ui = hbox(p1,p2) ### this does not
ERROR: StackOverflowError:

julia> WebIO.webio_serve(page("/", req -> ui), 5006)
```