fix no nucleus in mathtext

Author: zblz

lib/matplotlib/mathtext.py

@@ -2791,7 +2791,8 @@ def subsuper(self, s, loc, toks):
             # create new Hlist without kerning
             nucleus = Hlist(new_children, do_kern=False)
         else:
-            last_char.width = last_char._metrics.advance
+            if isinstance(nucleus, Char):
+                last_char.width = last_char._metrics.advance
             nucleus = Hlist([nucleus])
 
         # Handle regular sub/superscripts

lib/matplotlib/tests/test_mathtext.py

@@ -101,6 +101,7 @@
     r'$xyz^kx_kx^py^{p-2} d_i^jb_jc_kd x^j_i E^0 E^0_u$', # github issue #4873
     r'${xyz}^k{x}_{k}{x}^{p}{y}^{p-2} {d}_{i}^{j}{b}_{j}{c}_{k}{d} {x}^{j}_{i}{E}^{0}{E}^0_u$',
     r'${\int}_x^x x\oint_x^x x\int_{X}^{X}x\int_x x \int^x x \int_{x} x\int^{x}{\int}_{x} x{\int}^{x}_{x}x$',
+    r'testing$^{123}$',
 
 ]