Fix compiler crash caused by top level return (#12501)

compiler/semexprs.nim

@@ -1724,7 +1724,7 @@ proc semReturn(c: PContext, n: PNode): PNode =
   result = n
   checkSonsLen(n, 1, c.config)
   if c.p.owner.kind in {skConverter, skMethod, skProc, skFunc, skMacro} or
-      isClosureIterator(c.p.owner.typ):
+      (not c.p.owner.typ.isNil and isClosureIterator(c.p.owner.typ)):
     if n.sons[0].kind != nkEmpty:
       # transform ``return expr`` to ``result = expr; return``
       if c.p.resultSym != nil:

tests/misc/t12480.nim

@@ -0,0 +1,5 @@
+discard """
+  errormsg: "'return' not allowed here"
+"""
+
+return