Fix NPE (#348)

Author: wheelerlaw

src/main/java/com/networknt/schema/JsonSchema.java

@@ -95,7 +95,7 @@ private URI combineCurrentUriWithIds(URI currentUri, JsonNode schemaNode) {
             try {
                 return this.validationContext.getURIFactory().create(currentUri, id);
             } catch (IllegalArgumentException e) {
-                throw new JsonSchemaException(ValidationMessage.of(ValidatorTypeCode.ID.getValue(), ValidatorTypeCode.ID, id, currentUri.toString()));
+                throw new JsonSchemaException(ValidationMessage.of(ValidatorTypeCode.ID.getValue(), ValidatorTypeCode.ID, id, currentUri == null ? "null" : currentUri.toString()));
             }
         }
     }

src/test/java/com/networknt/schema/Issue347Test.java

@@ -0,0 +1,22 @@
+package com.networknt.schema;
+
+import com.fasterxml.jackson.databind.ObjectMapper;
+import org.junit.Test;
+import static org.junit.Assert.assertEquals;
+import static org.hamcrest.CoreMatchers.instanceOf;
+import static org.hamcrest.MatcherAssert.assertThat;
+
+public class Issue347Test {
+
+    @Test
+    public void failure() {
+        ObjectMapper mapper = new ObjectMapper();
+        JsonSchemaFactory factory = JsonSchemaFactory.getInstance(SpecVersion.VersionFlag.V7);
+        try {
+            JsonSchema schema = factory.getSchema(Thread.currentThread().getContextClassLoader().getResourceAsStream("schema/issue347-v7.json"));
+        } catch (Throwable e) {
+            assertThat(e, instanceOf(JsonSchemaException.class));
+            assertEquals("test: null is an invalid segment for URI {2}", e.getMessage());
+        }
+    }
+}

src/test/resources/schema/issue347-v7.json

@@ -0,0 +1,19 @@
+{
+  "$id": "test",
+  "$schema": "http://json-schema.org/draft-07/schema#",
+  "description": "Test description",
+  "properties": {
+    "test": {
+      "description": "Sample property",
+      "examples": [
+        "hello",
+        "world"
+      ],
+      "type": "string"
+    }
+  },
+  "required": [
+    "test"
+  ],
+  "type": "object"
+}