A rather nasty issue evaded our testing and found its way into IDA 7.6: using the PyQt5 modules that are shipped with IDA, QTreeView (or QTreeWidget) instances will always fail to display contents.
E.g., the following script
from PyQt5 import QtWidgets
tree = QtWidgets.QTreeWidget()
item = QtWidgets.QTreeWidgetItem()
item.setText(0, "Test col#0")
tree.addTopLevelItem(item)
tree.show()
used to render like so:
but now looks like this:
In order to solve this, please download the fix corresponding to your IDA installation, and replace the original QtWidgets DLL with the one contained in the .zip file