Zombie clips, enter/leave group?

[Steven Boswell II]

I'm using kdenlive 18.12.3 and mlt 6.12.0, as supplied by Fedora Core 29. Presently, I'm using kdenlive for the most complex project I've ever attempted, so I'm having issues I haven't had before.
Every once in a while -- and I don't know the reproduction steps -- one of my video clips will stop working. It still appears in the timeline, but it doesn't display any more, and won't export. I can't move it, and if I try to resize it, it disappears, and I get a message "Error resizing clip". I can reconstruct the clip from its original source, but it's a pain.
Also, is there any way to enter a group, so that the clips in there can be edited individually without having to ungroup them? I'm thinking of something similar to what LibreOffice Draw has with grouped shapes -- I can select a group, and enter it, and all other shapes are grayed out, and I can edit the contained shapes (or enter sub-groups). I've grouped my clips by scene, but if I want to edit something in an older scene, I have to move surrounding groups away from it, so that I can ungroup it, edit its clips, and then re-group them without the danger of including nearby clips I didn't intend.
Thanks for any help, and thank you very much for creating this editor. I wish I had time and energy outside of work to contribute programming help, but I have one of those more-than-full-time jobs.
Steven Boswell
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