I filed Bug 1602727 to fix other issues related to incorrect _destroyer patterns in Devtools, leading to target-mixin and toolbox destroy being called several times
Bug 1602700 Comment 3 Edit History
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I filed Bug 1602727 to fix other issues related to incorrect _destroyer patterns in Devtools, leading to target-mixin and toolbox destroy being called several times (and I just checked, fixing it will avoid calling webconsole front destroy twice on toolbox destroy)