diff --git a/app/src/processing/app/Mode.java b/app/src/processing/app/Mode.java
index 06b7bb057..71d31db9b 100644
--- a/app/src/processing/app/Mode.java
+++ b/app/src/processing/app/Mode.java
@@ -282,7 +282,7 @@ public abstract class Mode {
   public void rebuildLibraryList() {
     //new Exception("Rebuilding library list").printStackTrace(System.out);
     // reset the table mapping imports to libraries
-    importToLibraryTable = new HashMap<String, List<Library>>();
+    HashMap<String, List<Library>> importToLibraryTable = new HashMap<>();
 
     Library core = getCoreLibrary();
     if (core != null) {
@@ -323,6 +323,11 @@ public abstract class Mode {
     for (Library lib : contribLibraries) {
       lib.addPackageList(importToLibraryTable);
     }
+
+    // Make this Map thread-safe
+    this.importToLibraryTable = Collections.unmodifiableMap(importToLibraryTable);
+
+    base.getEditors().forEach(Editor::librariesChanged);
   }
 
 
diff --git a/java/src/processing/mode/java/pdex/ErrorCheckerService.java b/java/src/processing/mode/java/pdex/ErrorCheckerService.java
index ec5d2d4c3..6fa7c6b30 100644
--- a/java/src/processing/mode/java/pdex/ErrorCheckerService.java
+++ b/java/src/processing/mode/java/pdex/ErrorCheckerService.java
@@ -703,7 +703,7 @@ public class ErrorCheckerService {
         .filter(pckg -> !ignorableImport(pckg))
         .map(pckg -> {
           try {
-            return mode.getLibrary(pckg); // TODO: this may not be thread-safe
+            return mode.getLibrary(pckg);
           } catch (SketchException e) {
             return null;
           }