Fix [#22199] Cloth Cache Panel > Disk Cache doesn't work

Condition for this to work (.blend file must be saved) was poorly communicated 
in the UI (printfs are no good for this - ideally should use reports). Tweaked this a bit.

release/scripts/ui/properties_physics_common.py

@@ -20,6 +20,7 @@
 
 narrowui = 180
 
+import bpy
 
 def point_cache_ui(self, context, cache, enabled, particles, smoke):
     layout = self.layout
@@ -67,7 +68,10 @@ def point_cache_ui(self, context, cache, enabled, particles, smoke):
             sub = col.column()
             sub.enabled = enabled
             sub.prop(cache, "quick_cache")
-            col.prop(cache, "disk_cache")
+            
+            sub = col.column()
+            sub.enabled = bpy.data.file_is_saved
+            sub.prop(cache, "disk_cache")
             col.label(text=cache.info)