Fix T38359: Not a bug, but better to grey out Material selector of particles when using Object or Group render type (since we always use duplicated objects' materials in this case!).

Referenced by issue #38847, Crash when importing Collada file with bones.
Referenced by issue #38641, 3D manipulator widget is huge in files saved with later versions
Referenced by issue #38643, Frame labels are invisible with default theme
Referenced by issue #38638, False warning on solidify modifier with bevelled curves
Referenced by issue #38627, Scaling on normals with complex mesh causes lag
Referenced by issue #38524, Setting sky texture direction with the python console has no effect
Referenced by issue #38471, linked objects lose parents on saving or on loading
Referenced by issue #38388, Creating a new scene with full copy sets rigidbody collision shapes to box.
Referenced by issue #38394, Unwanted change user perspective (ortho to perspective) in 3d view
Referenced by issue #38362, Tab coloring broken
Referenced by issue #38359, Wrong material reported in particles settings when pinning them and selecting another object.

release/scripts/startup/bl_ui/properties_particle.py

@@ -800,6 +800,8 @@ class PARTICLE_PT_render(ParticleButtonsPanel, Panel):
         part = particle_get_settings(context)
 
         row = layout.row()
+        if part.render_type in {'OBJECT', 'GROUP'}:
+            row.enabled = False
         row.prop(part, "material_slot", text="")
         if psys:
             row.prop(psys, "parent")