[FIX] attribute-string-redundant: Add "isinstance" validation for nodes

Fix #109

pylint_odoo/checkers/no_modules.py

@@ -378,11 +378,15 @@ def visit_call(self, node):
         if node.as_string().lower().startswith('fields.'):
             args = misc.join_node_args_kwargs(node)
             index = 0
+            field_name = ''
+            if (isinstance(node.parent, astroid.Assign) and
+                    node.parent.targets and
+                    isinstance(node.parent.targets[0], astroid.AssignName)):
+                field_name = (node.parent.targets[0].name
+                              .replace('_', ' '))
             for argument in args:
                 argument_aux = argument
                 # Check this 'name = fields.Char("name")'
-                field_name = (argument.parent.parent.targets[0].name
-                              .replace('_', ' '))
                 if (isinstance(argument, astroid.Const) and
                     (index ==
                      FIELDS_METHOD.get(argument.parent.func.attrname, 0)) and
@@ -402,7 +406,8 @@ def visit_call(self, node):
                     # Check if the param string is equal to the name
                     #   of variable
                     elif argument.arg == 'string' and \
-                        (argument.value.value in
+                        (isinstance(argument_aux, astroid.Const) and
+                         argument_aux.value in
                          [field_name.capitalize(), field_name.title()]):
                         self.add_message(
                             'attribute-string-redundant', node=node)