gopls/internal/server: fix nil panic in findMatchingDiagnostics

Fixes golang/go#71028

Change-Id: Id669625f85066af1ee0c3e26bd557c5603bfb961
Reviewed-on: https://go-review.googlesource.com/c/tools/+/639396
LUCI-TryBot-Result: Go LUCI <[email protected]>
Auto-Submit: Alan Donovan <[email protected]>
Reviewed-by: Robert Findley <[email protected]>

Author: adonovan

gopls/internal/server/code_action.go

@@ -324,14 +324,18 @@ func (s *server) findMatchingDiagnostics(uri protocol.DocumentURI, pd protocol.D
 	defer s.diagnosticsMu.Unlock()
 
 	var sds []*cache.Diagnostic
-	for _, viewDiags := range s.diagnostics[uri].byView {
-		for _, sd := range viewDiags.diagnostics {
-			sameDiagnostic := (pd.Message == strings.TrimSpace(sd.Message) && // extra space may have been trimmed when converting to protocol.Diagnostic
-				protocol.CompareRange(pd.Range, sd.Range) == 0 &&
-				pd.Source == string(sd.Source))
+	if fileDiags := s.diagnostics[uri]; fileDiags != nil {
+		for _, viewDiags := range fileDiags.byView {
+			for _, sd := range viewDiags.diagnostics {
+				// extra space may have been trimmed when
+				// converting to protocol.Diagnostic
+				sameDiagnostic := pd.Message == strings.TrimSpace(sd.Message) &&
+					protocol.CompareRange(pd.Range, sd.Range) == 0 &&
+					pd.Source == string(sd.Source)
 
-			if sameDiagnostic {
-				sds = append(sds, sd)
+				if sameDiagnostic {
+					sds = append(sds, sd)
+				}
 			}
 		}
 	}