UI: Catch TypeError when uploading a config file and set appropriate error message

This error message is different from the one in sendConfigToServer()
since this one only checks for 3 things (metadata.encrypted,
configuration, credentials). The one in sendConfigToServer() tries
importing the whole configuration.

monkey/monkey_island/cc/ui/src/components/configuration-components/ImportConfigModal.tsx

@@ -142,9 +142,20 @@ const ConfigImportModal = (props: Props) => {
         setErrorMessage('File is not in a valid json format');
         return
       }
-      setConfigEncrypted(importContents['metadata']['encrypted']);
-      setConfigContents(importContents['configuration']);
-      setConfigCredentials(importContents['credentials']);
+
+      try {
+        setConfigEncrypted(importContents['metadata']['encrypted']);
+        setConfigContents(importContents['configuration']);
+        setConfigCredentials(importContents['credentials']);
+      } catch (e) {
+        if (e instanceof TypeError) {
+          setErrorMessage('Missing required fields, configuration file is most '
+          + 'likely from an old version')
+        }
+        else {
+          throw e;
+        }
+      }
     };
     reader.readAsText(event.target.files[0]);
   }