diff -rauN seamonkey-2.49.4/mozilla/js/xpconnect/src/XPCWrappedNative.cpp seamonkey-2.49.4-string-fmt-patch/mozilla/js/xpconnect/src/XPCWrappedNative.cpp
--- seamonkey-2.49.4/mozilla/js/xpconnect/src/XPCWrappedNative.cpp	2018-07-12 04:17:10.000000000 +0200
+++ seamonkey-2.49.4-string-fmt-patch/mozilla/js/xpconnect/src/XPCWrappedNative.cpp	2019-08-30 15:05:08.309875223 +0200
@@ -2219,7 +2219,7 @@
                                     "(%s" : (i == count-1) ?
                                         ", %s)" : ", %s";
                 name = JS_sprintf_append(name, fmt,
-                                         array[i]->GetNameString());
+                                         (array[i]->GetNameString() != NULL) ? array[i]->GetNameString() : "(NULL)");
             }
         }
     }