fix: handle nil error with concrete type

_assertauto/TestConfig/ErrorNil.json

@@ -0,0 +1,10 @@
+{
+	"entries": [
+		{
+			"equal": "(*pretty_test.testError) => <nil>"
+		},
+		{
+			"equal": 0
+		}
+	]
+}

ext/pierrreerrors/pierrreerrors.go

@@ -36,6 +36,9 @@ func write(c *pretty.Config, w io.Writer, st *pretty.State, v reflect.Value) boo
 	if !v.Type().Implements(typeError) {
 		return false
 	}
+	if v.Kind() == reflect.Pointer && v.IsNil() {
+		return false
+	}
 	err := v.Interface().(error) //nolint:forcetypeassert // Checked above.
 	st.Indent++
 	iw := pretty.GetIndentWriter(w, c, st, true)

ext/pierrreerrors/pierrreerrors_test.go

@@ -17,6 +17,15 @@ func TestValueWriter(t *testing.T) {
 	assert.StringHasPrefix(t, s, "(*errstack.stack) error\n\tstack\n")
 }
 
+func TestValueWriterNil(t *testing.T) {
+	c := pretty.NewConfig()
+	c.ValueWriters = nil
+	Configure(c)
+	var err error = (*testError)(nil)
+	s := c.String(err)
+	assert.Equal(t, s, "(*pierrreerrors.testError) => <nil>")
+}
+
 func TestValueWriterUnexported(t *testing.T) {
 	c := pretty.NewConfig()
 	c.ValueWriters = nil
@@ -27,6 +36,12 @@ func TestValueWriterUnexported(t *testing.T) {
 	assert.StringHasPrefix(t, s, "(*pierrreerrors.testUnexported) => (pierrreerrors.testUnexported) {\n\tv: (*errstack.stack) => (errstack.stack) {\n\t\terror: (*errors.errorString) => (errors.errorString)")
 }
 
+type testError struct{}
+
+func (e *testError) Error() string {
+	return "test"
+}
+
 type testUnexported struct {
 	v any
 }

pretty.go

@@ -600,6 +600,9 @@ func writeError(c *Config, w io.Writer, st *State, v reflect.Value) bool {
 	if !v.Type().Implements(typeError) {
 		return false
 	}
+	if v.Kind() == reflect.Pointer && v.IsNil() {
+		return false
+	}
 	err := v.Interface().(error) //nolint:forcetypeassert // Checked above.
 	_, _ = writeString(w, "=> .Error() => ")
 	_, _ = strconvio.WriteQuote(w, err.Error())

pretty_test.go

@@ -404,6 +404,13 @@ var testCases = []testCase{
 			c.ValueWriters = []ValueWriter{NewErrorValueWriter()}
 		},
 	},
+	{
+		name:  "ErrorNil",
+		value: (*testError)(nil),
+		configure: func(c *Config) {
+			c.ValueWriters = []ValueWriter{NewErrorValueWriter()}
+		},
+	},
 	{
 		name:  "ErrorUnexported",
 		value: testUnexported{v: &testError{}},