patch 9.0.1914: Vim9: few issues when accessing object members

Problem:  Vim9: few issues when accessing object members
Solution: When calling an object method, check for null object.
          Accessing a Dict object member doesn't work.

closes: #13119
closes: #13123
closes: #13124

Signed-off-by: Christian Brabandt <cb@256bit.org>
Co-authored-by: Yegappan Lakshmanan <yegappan@yahoo.com>
diff --git a/src/vim9class.c b/src/vim9class.c
index f122f90..a2e305c 100644
--- a/src/vim9class.c
+++ b/src/vim9class.c
@@ -534,7 +534,7 @@
 	for (int i = 0; i < extends_method_count; i++)
 	{
 	    ufunc_T *uf = extends_methods[i];
-	    if ((uf->uf_flags & FC_ABSTRACT) == 0)
+	    if (!IS_ABSTRACT_METHOD(uf))
 		continue;
 
 	    int method_found = FALSE;