Bugfix/customproxy (#2738)

* BugFix #2737
* Added a CustomProxy class which overrides the invocation handler to return approproate values for different datatypes

(cherry picked from commit f68d2e880074277b75a60b730dd63254239a8971)

* Added JavaDocs for CustomProxy

app/src/main/java/fr/free/nrw/commons/utils/CustomProxy.java

@@ -0,0 +1,33 @@
+package fr.free.nrw.commons.utils;
+
+import java.lang.reflect.InvocationHandler;
+import java.lang.reflect.Proxy;
+
+/**
+ * Returns a new instance of proxy with overriden invocationhanlder() returning appropriate values
+ * for different datatypes
+ * See https://stackoverflow.com/questions/52083338/expected-to-unbox-a-string-primitive-type-but-was-returned-null
+ */
+public class CustomProxy extends Proxy {
+    protected CustomProxy(InvocationHandler h) {
+        super(h);
+    }
+
+    public static Object newInstance(ClassLoader loader, Class<?>[] interfaces) {
+        return Proxy.newProxyInstance(loader, interfaces, (o, method, objects) -> {
+            if (String.class == method.getReturnType()) {
+                return "";
+            } else if (Integer.class == method.getReturnType()) {
+                return Integer.valueOf(0);
+            } else if (int.class == method.getReturnType()) {
+                return 0;
+            } else if (Boolean.class == method.getReturnType()) {
+                return Boolean.FALSE;
+            } else if (boolean.class == method.getReturnType()) {
+                return false;
+            } else {
+                return null;
+            }
+        });
+    }
+}