diff --git a/CHANGELOG.md b/CHANGELOG.md
index a1c700c2..ad4f38bb 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,6 +1,16 @@
 FlatLaf Change Log
 ==================
 
+## 3.5-SNAPSHOT
+
+#### Incompatibilities
+
+- ProgressBar: Log warning (including stack trace) when uninstalling
+  indeterminate progress bar UI or using JProgressBar.setIndeterminate(false)
+  not on AWT thread, because this may throw NPE in FlatProgressBarUI.paint().
+  (issues #841 and #830)
+
+
 ## 3.4.1
 
 #### Fixed bugs
diff --git a/flatlaf-core/src/main/java/com/formdev/flatlaf/ui/FlatProgressBarUI.java b/flatlaf-core/src/main/java/com/formdev/flatlaf/ui/FlatProgressBarUI.java
index 58d02190..6e22376d 100644
--- a/flatlaf-core/src/main/java/com/formdev/flatlaf/ui/FlatProgressBarUI.java
+++ b/flatlaf-core/src/main/java/com/formdev/flatlaf/ui/FlatProgressBarUI.java
@@ -18,6 +18,7 @@ package com.formdev.flatlaf.ui;
 
 import static com.formdev.flatlaf.FlatClientProperties.*;
 import java.awt.Dimension;
+import java.awt.EventQueue;
 import java.awt.FontMetrics;
 import java.awt.Graphics;
 import java.awt.Graphics2D;
@@ -86,6 +87,17 @@ public class FlatProgressBarUI
 		installStyle();
 	}
 
+	@Override
+	public void uninstallUI( JComponent c ) {
+		if( !EventQueue.isDispatchThread() && progressBar.isIndeterminate() ) {
+			LoggingFacade.INSTANCE.logSevere(
+				"FlatLaf: Uninstalling indeterminate progress bar UI not on AWT thread may throw NPE in FlatProgressBarUI.paint(). Use SwingUtilities.invokeLater().",
+				new IllegalStateException() );
+		}
+
+		super.uninstallUI( c );
+	}
+
 	@Override
 	protected void installDefaults() {
 		super.installDefaults();
@@ -110,6 +122,14 @@ public class FlatProgressBarUI
 
 		propertyChangeListener = e -> {
 			switch( e.getPropertyName() ) {
+				case "indeterminate":
+					if( !EventQueue.isDispatchThread() && !progressBar.isIndeterminate() ) {
+						LoggingFacade.INSTANCE.logSevere(
+							"FlatLaf: Using JProgressBar.setIndeterminate(false) not on AWT thread may throw NPE in FlatProgressBarUI.paint(). Use SwingUtilities.invokeLater().",
+							new IllegalStateException() );
+					}
+					break;
+
 				case PROGRESS_BAR_LARGE_HEIGHT:
 				case PROGRESS_BAR_SQUARE:
 					progressBar.revalidate();