account for nil return type in tyProc sumGeneric (#23250)

fixes #23249

compiler/sigmatch.nim

@@ -246,7 +246,8 @@ proc sumGeneric(t: PType): int =
           result += sumGeneric(a)
       break
     of tyProc:
-      result += sumGeneric(t.returnType)
+      if t.returnType != nil:
+        result += sumGeneric(t.returnType)
       for _, a in t.paramTypes:
         result += sumGeneric(a)
       break

tests/overload/t23249.nim

@@ -0,0 +1,17 @@
+# issue #23249
+
+type Control* = object
+proc onAction*(c: Control, handler: proc(e: int) {.gcsafe.}) = discard
+proc onAction*(c: Control, handler: proc() {.gcsafe.}) = discard
+
+template setControlHandlerBlock(c: Control, p: untyped, a: untyped) =
+    when compiles(c.p(nil)):
+        c.p() do() {.gcsafe.}: a
+    else:
+        c.p = proc() {.gcsafe.} =
+            a
+
+proc mkLayout() =
+  var b: Control
+  setControlHandlerBlock(b, onAction):
+    echo "hi"