Fix null pointer exception in ExprBlocks when using the ExprDirection…

… without a number defined. (#5790)

Update ExprBlocks.java

(cherry picked from commit 549ebc3)

src/main/java/ch/njol/skript/expressions/ExprBlocks.java

@@ -151,9 +151,11 @@ public Iterator<Block> iterator(Event event) {
 				int distance = SkriptConfig.maxTargetBlockDistance.value();
 				if (this.direction instanceof ExprDirection) {
 					Expression<Number> numberExpression = ((ExprDirection) this.direction).amount;
-					Number number = numberExpression.getSingle(event);
-					if (numberExpression != null && number != null)
-						distance = number.intValue();
+					if (numberExpression != null) {
+						Number number = numberExpression.getSingle(event);
+						if (number != null)
+							distance = number.intValue();
+					}
 				}
 				return new BlockLineIterator(location, vector, distance);
 			} else {