In all of my plugs I get recently this warning:

[FAIL]  UI Instantiation
              failed to instantiate
              seeAlso: <http://lv2plug.in/ns/extensions/ui#X11UI>

while my plugs itself print the following warning: ERROR: Failed to open parentXwindow

So, yes,

if (!strcmp(features[i]->URI, LV2_UI__parent)) {
            ui->parentXwindow = features[i]->data;
}

fail, my plugs don't create the UI and return NULL, intentional, as they need a parent widget to be embedded in.

using lv2lint.bin 0.17.61