Fix NPE when Collection contains unbound children

Fixes gh-13636

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/context/properties/bind/IndexedElementsBinder.java

@@ -128,9 +128,10 @@ abstract class IndexedElementsBinder<T> extends AggregateBinder<T> {
 		}
 		for (ConfigurationPropertyName name : (IterableConfigurationPropertySource) source
 				.filter(root::isAncestorOf)) {
-			name = name.chop(root.getNumberOfElements() + 1);
-			if (name.isLastElementIndexed()) {
-				String key = name.getLastElement(Form.UNIFORM);
+			ConfigurationPropertyName choppedName = name
+					.chop(root.getNumberOfElements() + 1);
+			if (choppedName.isLastElementIndexed()) {
+				String key = choppedName.getLastElement(Form.UNIFORM);
 				ConfigurationProperty value = source.getConfigurationProperty(name);
 				children.add(key, value);
 			}

spring-boot-project/spring-boot/src/test/java/org/springframework/boot/context/properties/bind/CollectionBinderTests.java

@@ -134,6 +134,29 @@ public class CollectionBinderTests {
 		}
 	}
 
+	@Test
+	public void bindToNonScalarCollectionWhenNonSequentialShouldThrowException() {
+		MockConfigurationPropertySource source = new MockConfigurationPropertySource();
+		source.put("foo[0].value", "1");
+		source.put("foo[1].value", "2");
+		source.put("foo[4].value", "4");
+		this.sources.add(source);
+		try {
+			Bindable<List<JavaBean>> target = Bindable.listOf(JavaBean.class);
+			this.binder.bind("foo", target);
+			fail("No exception thrown");
+		}
+		catch (BindException ex) {
+			ex.printStackTrace();
+			Set<ConfigurationProperty> unbound = ((UnboundConfigurationPropertiesException) ex
+					.getCause()).getUnboundProperties();
+			assertThat(unbound).hasSize(1);
+			ConfigurationProperty property = unbound.iterator().next();
+			assertThat(property.getName().toString()).isEqualTo("foo[4].value");
+			assertThat(property.getValue()).isEqualTo("4");
+		}
+	}
+
 	@Test
 	public void bindToCollectionWhenNonIterableShouldReturnPopulatedCollection() {
 		MockConfigurationPropertySource source = new MockConfigurationPropertySource();