fix: Nil pointer when getting context's flagSet(#1325) (#1327)

* fix: Nil pointer when getting context's flagSet in UsageError function(#1325)

* test: add unit test for nil flagset in Context

context.go

@@ -151,6 +151,9 @@ func (ctx *Context) lookupFlag(name string) Flag {
 
 func (ctx *Context) lookupFlagSet(name string) *flag.FlagSet {
 	for _, c := range ctx.Lineage() {
+		if c.flagSet == nil {
+			continue
+		}
 		if f := c.flagSet.Lookup(name); f != nil {
 			return c.flagSet
 		}

context_test.go

@@ -112,6 +112,8 @@ func TestContext_String(t *testing.T) {
 	c := NewContext(nil, set, parentCtx)
 	expect(t, c.String("myflag"), "hello world")
 	expect(t, c.String("top-flag"), "hai veld")
+	c = NewContext(nil, nil, parentCtx)
+	expect(t, c.String("top-flag"), "hai veld")
 }
 
 func TestContext_Path(t *testing.T) {