promtail: Handle nil error on target Details() call (#7771)

**What this PR does / why we need it**:
This PR addresses issue [#6930](#6930).
Promtail should not panic when the Details() method is called for a target that has no errors.

**Which issue(s) this PR fixes**:
Fixes #6930

CHANGELOG.md

@@ -33,6 +33,7 @@
 * [7602](https://github.com/grafana/loki/pull/7602) **vmax**: Add decolorize stage to Promtail to easily parse colored logs.
 
 ##### Fixes
+* [7771](https://github.com/grafana/loki/pull/7771) **GeorgeTsilias**: Handle nil error on target Details() call.
 
 * [7461](https://github.com/grafana/loki/pull/7461) **MarNicGit**: Promtail: Fix collecting userdata field from Windows Event Log
 

clients/pkg/promtail/targets/cloudflare/target.go

@@ -224,9 +224,13 @@ func (t *Target) Ready() bool {
 
 func (t *Target) Details() interface{} {
 	fields, _ := Fields(FieldsType(t.config.FieldsType))
+	var errMsg string
+	if t.err != nil {
+		errMsg = t.err.Error()
+	}
 	return map[string]string{
 		"zone_id":        t.config.ZoneID,
-		"error":          t.err.Error(),
+		"error":          errMsg,
 		"position":       t.positions.GetString(positions.CursorKey(t.config.ZoneID)),
 		"last_timestamp": t.to.String(),
 		"fields":         strings.Join(fields, ","),

clients/pkg/promtail/targets/docker/target.go

@@ -251,9 +251,13 @@ func (t *Target) Labels() model.LabelSet {
 
 // Details returns target-specific details.
 func (t *Target) Details() interface{} {
+	var errMsg string
+	if t.err != nil {
+		errMsg = t.err.Error()
+	}
 	return map[string]string{
 		"id":       t.containerName,
-		"error":    t.err.Error(),
+		"error":    errMsg,
 		"position": t.positions.GetString(positions.CursorKey(t.containerName)),
 		"running":  strconv.FormatBool(t.running.Load()),
 	}