Fix contributed upstream here:
https://debbugs.gnu.org/cgi/bugreport.cgi?bug=42694.

Index: emacs-26.3/src/xfns.c
===================================================================
--- emacs-26.3.orig/src/xfns.c
+++ emacs-26.3/src/xfns.c
@@ -2628,6 +2628,8 @@ xic_free_xfontset (struct frame *f)
 static XIMStyle
 best_xim_style (XIMStyles *xim)
 {
+  if (xim == NULL) goto out;
+
   int i, j;
   int nr_supported = ARRAYELTS (supported_xim_styles);

@@ -2636,6 +2638,7 @@ best_xim_style (XIMStyles *xim)
       if (supported_xim_styles[i] == xim->supported_styles[j])
	return supported_xim_styles[i];

+ out:
   /* Return the default style.  */
   return XIMPreeditNothing | XIMStatusNothing;
 }
();'>
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<tr><td><span title='2018-04-19 00:14:49 +0200'>2018-04-19</span></td><td><a href='/guix/commit/po/packages/es.po?id=c88abbf2fda8f3798a0e3dd14a02157d247875e8'>nls: Update 'es' translation.</a></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2018-04-11 01:21:32 +0200'>2018-04-11</span></td><td><a href='/guix/commit/po/packages/es.po?id=ac6d2e35300012ddac2eea13b09e7a20cfb02520'>nls: Add Spanish translation.</a></td><td>Ludovic Courtès</td></tr>