Index: /trunk/test/unit/org/openstreetmap/josm/gui/TableCellRendererTest.java
===================================================================
--- /trunk/test/unit/org/openstreetmap/josm/gui/TableCellRendererTest.java	(revision 10020)
+++ /trunk/test/unit/org/openstreetmap/josm/gui/TableCellRendererTest.java	(revision 10020)
@@ -0,0 +1,112 @@
+// License: GPL. For details, see LICENSE file.
+package org.openstreetmap.josm.gui;
+
+import java.lang.reflect.Constructor;
+import java.lang.reflect.InvocationTargetException;
+import java.lang.reflect.Modifier;
+import java.util.Arrays;
+import java.util.Collection;
+import java.util.Set;
+
+import javax.swing.JTable;
+import javax.swing.table.TableCellRenderer;
+
+import org.junit.Assert;
+import org.junit.BeforeClass;
+import org.junit.Test;
+import org.openstreetmap.josm.JOSMFixture;
+import org.reflections.Reflections;
+
+/**
+ * Checks if all classes implementing the {@link TableCellRenderer} interface do
+ * accept a null value as second parameter for
+ * {@link TableCellRenderer#getTableCellRendererComponent(javax.swing.JTable,
+ * java.lang.Object, boolean, boolean, int, int)}.
+ *
+ * For unknown reason java sometimes call getTableCellRendererComponent method
+ * with value = null. Every implementation of {@code getTableCellRendererComponent}
+ * must fail gracefully when null is passed as value parameter.
+ *
+ * This test scans the classpath for classes implementing {@code TableCellRenderer},
+ * creates an instance and calls {@code getTableCellRendererComponent} with null
+ * value to check if a NPE is thrown.
+ *
+ * @see #6301
+ */
+public class TableCellRendererTest {
+
+    // list of classes that cannot be easily tested and are verified manually
+    private static final Collection<String> SKIP_TEST = Arrays.asList(
+        "org.openstreetmap.josm.gui.dialogs.FilterDialog$BooleanRenderer"
+    );
+
+    /**
+     * Setup test.
+     */
+    @BeforeClass
+    public static void setUp() {
+        JOSMFixture.createFunctionalTestFixture().init(true);
+    }
+
+    @Test
+    public void tableCellRendererTest() {
+        Reflections reflections = new Reflections("org.openstreetmap.josm");
+        Set<Class<? extends TableCellRenderer>> renderers = reflections.getSubTypesOf(TableCellRenderer.class);
+        Assert.assertTrue(renderers.size() >= 10); // if it finds less than 10 classes, something is broken
+        JTable tbl = new JTable(2, 2);
+        for (Class<? extends TableCellRenderer> klass : renderers) {
+            if (Modifier.isAbstract(klass.getModifiers()) || SKIP_TEST.contains(klass.getName())) {
+                continue;
+            }
+            if (klass.isAnonymousClass()) {
+                continue;
+            }
+            TableCellRenderer tcr = createInstance(klass);
+            try {
+                tcr.getTableCellRendererComponent(tbl, null, false, false, 0, 0);
+            } catch (NullPointerException npe) {
+                npe.printStackTrace();
+                Assert.fail("NPE in getTableCellRendererComponent");
+            }
+        }
+    }
+
+    /**
+     * Create an instance of a class assuming it has a no-args constructor.
+     * @param <T> the class or a super-type of the class
+     * @param klass the class
+     * @return an instance of the class
+     */
+    private static <T> T createInstance(Class<? extends T> klass) {
+        boolean needOuterClass = klass.isMemberClass() && !Modifier.isStatic(klass.getModifiers());
+        Constructor<? extends T> c;
+        try {
+            if (needOuterClass) {
+                c = klass.getDeclaredConstructor(klass.getDeclaringClass());
+            } else {
+                c = klass.getDeclaredConstructor();
+            }
+        } catch (NoSuchMethodException ex) {
+            // no default constructor - to fix this, add a default constructor
+            // to the class or add the class to the SKIP_TEST list above
+            Assert.fail("No default constructor - cannot test TableCellRenderer: " + ex);
+            return null;
+        } catch (SecurityException ex) {
+            throw new RuntimeException(ex);
+        }
+        c.setAccessible(true);
+        T o;
+        try {
+            if (needOuterClass) {
+                Object outerInstance = createInstance(klass.getDeclaringClass());
+                o = c.newInstance(outerInstance);
+            } else {
+                o = c.newInstance();
+            }
+        } catch (InstantiationException | IllegalArgumentException | IllegalAccessException | InvocationTargetException ex) {
+            throw new RuntimeException(ex);
+        }
+        return o;
+    }
+
+}