Fix reading multiple parameter subset files from YAML (#157)

Author: dilpath

petab/problem.py

@@ -217,10 +217,10 @@ def from_yaml(yaml_config: Union[Dict, Path, str]) -> 'Problem':
                 'Support for multiple models is not yet implemented.')
 
         if isinstance(yaml_config[PARAMETER_FILE], list):
-            parameter_df = [
-                parameters.get_parameter_df(get_path(f))
+            parameter_df = parameters.get_parameter_df([
+                get_path(f)
                 for f in yaml_config[PARAMETER_FILE]
-            ]
+            ])
         else:
             parameter_df = parameters.get_parameter_df(
                 get_path(yaml_config[PARAMETER_FILE])) \