JuliaGtk failure when called from Timer callback

For a course I’m writing on Julia + Gtk, I want to include an alert function that waits for a user-specified time and then displays a message, converted from my working Python version. The Julia version fails, and I’ve included a minimum working example of the failure. The function test() draws a rectangle when called from the button click callback, but stops the timer callback process(t) when called from process, as can be demonstrated by changing the variable autocall to true. This sounds like a stack management error. I’ll be grateful for any ideas on how to make this work.
Macbook Pro OS 10.14, Julia 1.5.3 and 1.0.3

# alertfaildemo.jl
# The function process() prints the time every second.
# When the button is clicked, the function test() draws a rectangle.
# If autocall is set true, the timer call fails and the autoprint stops

using Gtk, Dates

autocall = false # Fails if this is changed to true

holdsecs = 1.0 # Wait time between time displays
status = 0 # 0, 1 for off, on
tprevdis = 0. # Time of previous display

button = GtkButton("Press to test")
win = GtkWindow("Draw Failure Demo")
canvas = @GtkCanvas(400, 50)
set_gtk_property!(canvas, :expand, true)
g = GtkGrid()
push!(win, g)
g[1,1] = button
g[1,2] = canvas

function onbuttonclick(button)
  global status
  status = 1 - status
  label = "Run"
  if status == 0
    label = "Stop"
  Gtk.GAccessor.label(button, label)
  println("button clicked, status=$status")
end # onbuttonclick

id = signal_connect(onbuttonclick, button, "clicked")

function test()
  println("test called")
  set_source_rgb(ctx, rand(), rand(), rand()) # random color on each call
  rectangle(ctx, 0, 0, width(canvas)/2, height(canvas)/2)

function process(t)
  global tprevdis
  tnow = time() # since the epoch
  if tnow - tprevdis < holdsecs || status == 1 return end
  if autocall
    test() # Stops future calls
  println(Dates.format(Dates.now(), Dates.RFC1123Format))
  tprevdis = tnow

t = Timer(process, 0, interval = 0.5) # Check every half sec