REFACTOR: Fields init arg name

The error would probably not have appeared if
the __iniy__ method of Fields had "post_app" as input
argument instead of "app".
I assume this led to the confusion with providing an AEDT app
such as Hfss, ...

src/ansys/aedt/core/visualization/report/field.py

@@ -67,10 +67,13 @@ def _context(self):
 
 
 class Fields(CommonReport):
-    """Provides for managing fields."""
+    """Handler to manage fields."""
 
-    def __init__(self, app, report_category, setup_name, expressions=None):
-        CommonReport.__init__(self, app, report_category, setup_name, expressions)
+    @pyaedt_function_handler(
+        app="post_app",
+    )
+    def __init__(self, post_app, report_category, setup_name, expressions=None):
+        CommonReport.__init__(self, post_app, report_category, setup_name, expressions)
         self.domain = "Sweep"
         self.primary_sweep = "Distance"