patch 8.2.1820: Vim9: crash when error happens in timer callback

Problem:    Vim9: crash when error happens in timer callback.
Solution:   Check that current_exception is not NULL. (closes #7100)
diff --git a/src/version.c b/src/version.c
index a953c09..8e2115e 100644
--- a/src/version.c
+++ b/src/version.c
@@ -751,6 +751,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    1820,
+/**/
     1819,
 /**/
     1818,