Fix #141808: Crash when a FileHandler uses an operator with no filepath

Add a null check and error in the UI.

Ref !141850

source/blender/editors/interface/templates/interface_templates.cc

@@ -3014,10 +3014,22 @@ static void draw_export_properties(bContext *C,
   uiLayoutSetPropDecorate(col, false);
 
   PropertyRNA *prop = RNA_struct_find_property(op->ptr, "filepath");
-  std::string placeholder = "//" + filename;
-  uiItemFullR(
-      col, op->ptr, prop, RNA_NO_INDEX, 0, UI_ITEM_NONE, nullptr, ICON_NONE, placeholder.c_str());
-
+  if (prop) {
+    std::string placeholder = "//" + filename;
+    uiItemFullR(col,
+                op->ptr,
+                prop,
+                RNA_NO_INDEX,
+                0,
+                UI_ITEM_NONE,
+                nullptr,
+                ICON_NONE,
+                placeholder.c_str());
+  }
+  else {
+    std::string label = std::string(IFACE_("<filepath> not found in: ")) + op->idname;
+    uiItemL(col, label.c_str(), ICON_ERROR);
+  }
   template_operator_property_buts_draw_single(
       C, op, layout, UI_BUT_LABEL_ALIGN_NONE, UI_TEMPLATE_OP_PROPS_HIDE_PRESETS);
 }