edit_roi_counters: image is not always displayed

Steps to reproduce the problem:

• start bliss -s test_session with a running LimaCCDS simulator
• config.get("lima_simulator")
• edit_roi_counters(lima_simulator)

Flint starts, and displays a white image (== no image).

The following sequence works:

• start bliss -s test_session with a running LimaCCDS simulator
• in shell: config.get("lima_simulator")
• in shell: ct(0.1, lima_simulator)
• edit_roi_counters(lima_simulator)