Return an error if the placeholder export are empty or no children collection.

Update the docstring operator to be more clear.

Fange_Pipeline/operators/exports.py

@@ -5,7 +5,8 @@ from pathlib import Path
 
 
 class ExportForFange(bpy.types.Operator):
-    """Export a building for fange"""
+    """Export a building for fange.
+Requiert your blend file are write on your hard-drive."""
     bl_idname = 'graou.building_export'
     bl_label = 'Easily export a building asset'
 
@@ -37,7 +38,12 @@ class ExportForFange(bpy.types.Operator):
             self.report({'ERROR'}, 'Can\'t find the asset category.')
             return {'CANCELLED'}
 
-        if len(self.coll_layout.collection_children):
+        if not len(self.coll_layout.children):
+            self.report({'WARNING'}, 'No children collection, nothing can be exported.')
+            return {'CANCELLED'}
+
+        if not len(self.coll_layout.collection_children):
+
             for coll in self.coll_layout.children:
                 print(f'[Pipeline] Update "{coll.name}" mesh')
                 child = bpy.data.collections.get(coll.name)