Fix a logistic bug
diff --git a/manimlib/mobject/svg/string_mobject.py b/manimlib/mobject/svg/string_mobject.py
index 5004960e61..e4376a3360 100644
--- a/manimlib/mobject/svg/string_mobject.py
+++ b/manimlib/mobject/svg/string_mobject.py
@@ -174,8 +174,8 @@ def find_spans_by_single_selector(sel):
             ):
                 l = self.full_span[1]
                 span = tuple(
+                    default_index if index is None else
                     min(index, l) if index >= 0 else max(index + l, 0)
-                    if index is not None else default_index
                     for index, default_index in zip(sel, self.full_span)
                 )
                 return [span]

## Motivation
There's a logistic bug when handling `None` in spans. My apologies for that.

## Proposed changes
- M `manimlib/mobject/svg/string_mobject.py`
