fix none errors

gen_seq_diag.py

@@ -178,6 +178,8 @@ class Call(Stmt):
                 return
 
         # check for recursion
+        id_ = None
+        if met2 is not None:
             id_ = (met2.t.name, met2.name)
         if id_ in ctx.vis:
             ctx.comment(lvl, f'recur {call.qual}.{call.member}')
@@ -211,6 +213,7 @@ class Call(Stmt):
                    f'dt=7' +
                    ']' +
                    f'{{{edge[0]}}}{{{edge[1]}}}')
+        if met2 is not None:
             met2.gen_seq(var, lvl + 1, ctx)
         ctx.append(lvl, '\\end{umlcall}')