by yunge.fu:

In pkg/html/template/template.go, the Parse() func do not assign value to t.Tree, which
just be added recently, so it's always nil.

func (t *Template) Parse(src string) (*Template, error) {
        t.nameSpace.mu.Lock()
        t.escaped = false
        t.nameSpace.mu.Unlock()
        ret, err := t.text.Parse(src)
        if err != nil {
                return nil, err
        }
        // In general, all the named templates might have changed underfoot.
        // Regardless, some new ones may have been defined.
        // The template.Template set has been updated; update ours.
        t.nameSpace.mu.Lock()
        defer t.nameSpace.mu.Unlock()
        for _, v := range ret.Templates() {
                name := v.Name()
                tmpl := t.set[name]
                if tmpl == nil {
                        tmpl = t.new(name)
                }
                tmpl.escaped = false
                tmpl.text = v
        }
        return t, nil
}