Fix config.get for iterable sections

src/sqlfluff/core/config.py

@@ -615,7 +615,11 @@ def get(
         self, val: str, section: Union[str, Iterable[str]] = "core", default: Any = None
     ):
         """Get a particular value from the config."""
-        return self._configs[section].get(val, default)
+        section_dict = self.get_section(section)
+        if section_dict is None:
+            return default
+
+        return section_dict.get(val, default)
 
     def get_section(self, section: Union[str, Iterable[str]]) -> Union[dict, None]:
         """Return a whole section of config as a dict.