Crash when reading XYZ Features

**osgEarth Version (required):**
 3.8.1 build 187

**Description of the problem:**
Crash if a XYZ Feature is present.

**What you have tried:**
Read MVT Features as XYZ Feature in earth file

**Screenshot, code block, or data file that will help reproduce the issue:**
osgearth_viewer osm.earth
