Ensure policy events do not cause a panic when using preview --json (#5610)

CHANGELOG.md

@@ -4,6 +4,10 @@ CHANGELOG
 ## HEAD (Unreleased)
 - [cli] Ensure that the CLI doesn't panic when using pulumi watch and policies are enabled
   [#5569](https://github.com/pulumi/pulumi/pull/5569)
+  
+- [cli] Ensure that the CLI doesn't panic when using the JSON output as part of previews 
+  and policies are enabled
+  [#5610](https://github.com/pulumi/pulumi/pull/5610)
 
 
 ## 2.12.0 (2020-10-14)

pkg/backend/display/json.go

@@ -181,6 +181,9 @@ func ShowJSONEvents(op string, action apitype.UpdateKind, events <-chan engine.E
 			// need to come up with a scheme for matching the failure to the associated step.
 
 		// Events ocurring late:
+		case engine.PolicyViolationEvent:
+			// At this point in time, we don't handle policy events in JSON serialization
+			continue
 		case engine.SummaryEvent:
 			// At the end of the preview, a summary event indicates the final conclusions.
 			p := e.Payload().(engine.SummaryEventPayload)