Hi, The panel-properties-dialog-gtk3.ui file is from 1.16. It was there when mate-panel supported both GTK+2 and GTK+3 builds. In 1.18 it's only GTK+3, and the file is named panel-properties-dialog.ui.
Both mate-panel executable and the mentioned .ui file are in the same package (mate-panel). If the panel tries to open the non-existent -gtk3.ui file, that means that somehow the package got corrupted, and you have mate-panel executable from 1.16 but .ui file from 1.18. Or maybe some other corruption happened. I suggest reinstalling the package: sudo apt-get install --reinstall mate-panel
