fix: avoid NPE when ConstantTimer.delay accesses a variable

The variable might not yet be initialized at the first iteration

Fixes #6162

src/components/src/main/java/org/apache/jmeter/timers/ConstantTimer.java

@@ -29,14 +29,12 @@
  * value update and user interaction.
  *
  */
-public class ConstantTimer extends AbstractTestElement implements Timer, Serializable, LoopIterationListener {
+public class ConstantTimer extends AbstractTestElement implements Timer, Serializable {
 
     private static final long serialVersionUID = 240L;
 
     public static final String DELAY = "ConstantTimer.delay"; //$NON-NLS-1$
 
-    private long delay = 0;
-
     /**
      * No-arg constructor.
      */
@@ -85,7 +83,7 @@ public double getRange() {
      */
     @Override
     public long delay() {
-        return delay;
+        return getPropertyAsLong(DELAY);
     }
 
     /**
@@ -97,15 +95,4 @@ public long delay() {
     public String toString() {
         return JMeterUtils.getResString("constant_timer_memo"); //$NON-NLS-1$
     }
-
-    /**
-     * Gain access to any variables that have been defined.
-     *
-     * @see LoopIterationListener#iterationStart(LoopIterationEvent)
-     */
-    @Override
-    public void iterationStart(LoopIterationEvent event) {
-        delay = getPropertyAsLong(DELAY);
-
-    }
 }