Yeelight Group Issue

[HydrelioxGitHub]

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Home Assistant release (hass --version): 0.38.2

**Python release (python3 --version):**3.4.2

**Component/platform:**Light/Yeelight/Group

Description of problem:
I have a group with all my yeelight bulb connected (group.light). When I toogle on/off each bulb individually, no problem. When I try to switch on or off all the group, and if one of the them is unavailable, only some of them are switching.

Expected:
All bulbs of the group switch on or off

Problem-relevant configuration.yaml entries and steps to reproduce:

A config with several bulbs on the same group and one of them unavailable (for exemple when they are physically switched off)