fixed import

Author: denizkilic

api/pom.xml

@@ -108,6 +108,11 @@
             <version>2.21.0</version>
             <scope>test</scope>
         </dependency>
+        <dependency>
+            <groupId>org.jetbrains</groupId>
+            <artifactId>annotations</artifactId>
+            <version>13.0</version>
+        </dependency>
     </dependencies>
 
     <distributionManagement>

api/src/main/java/com/messagebird/objects/MessageResponse.java

@@ -1,6 +1,6 @@
 package com.messagebird.objects;
 
-import com.sun.istack.internal.Nullable;
+import org.jetbrains.annotations.Nullable;
 
 import java.io.Serializable;
 import java.math.BigInteger;