Missing icon for Mail Reader in application menu

[Paul Menzel]

Dear Xfce folks,


With Xfce 4.12, there is no icon shown for the *Mail Reader* in the 
application menu. Please find a screenshot attached.

Mozilla Thunderbird is used as the mail reader here by default, and its 
icon is correctly shown under the section *Internet*.

But I believe, the “generic” “Mail Reader” icon is missing here somehow. 
Could you please tell me, where this is configured. Adwaita 3.26.0 is 
selected as the theme for Xfce.


Kind regards,

Paul
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