Fix IllegalArgumentException for File properties which allow null value

src/core/org/apache/jmeter/testbeans/gui/FieldStringEditor.java

@@ -27,7 +27,7 @@
 
 
 /**
- * This class implements a property editor for non-null String properties that
+ * This class implements a property editor for possible null String properties that
  * supports custom editing (i.e.: provides a GUI component) based on a text
  * field.
  * <p>
@@ -47,14 +47,15 @@ class FieldStringEditor extends PropertyEditorSupport implements ActionListener,
      * Value on which we started the editing. Used to avoid firing
      * PropertyChanged events when there's not been such change.
      */
-    private String initialValue = "";
+    private String initialValue;
 
     protected FieldStringEditor() {
         super();
 
         textField = new JTextField();
         textField.addActionListener(this);
         textField.addFocusListener(this);
+        initialValue = textField.getText();
     }
 
     @Override
@@ -75,10 +76,11 @@ public Object getValue() {
 
     @Override
     public void setValue(Object value) {
-        if (value instanceof String) {
+        // null value always fail instanceof check therefore we process it separately
+        if (value == null || value instanceof String) {
             setAsText((String) value);
         } else {
-            throw new IllegalArgumentException();
+            throw new IllegalArgumentException("Expected String but got "+value.getClass()+", value="+value);
         }
     }
 
@@ -99,7 +101,9 @@ public Component getCustomEditor() {
     public void firePropertyChange() {
         String newValue = getAsText();
 
-        if (initialValue.equals(newValue)) {
+        if (initialValue != null && initialValue.equals(newValue)
+         || initialValue == null && newValue == null
+        ) {
             return;
         }
         initialValue = newValue;