Improve error message on NPE in on

Author: nhaarman

mockito-kotlin/src/main/kotlin/com/nhaarman/mockitokotlin2/Mockito.kt

@@ -215,7 +215,7 @@ class KStubbing<out T>(private val mock: T) {
         return try {
             Mockito.`when`(mock.methodCall())
         } catch (e: NullPointerException) {
-            throw MockitoKotlinException("NullPointerException thrown when stubbing. If you are trying to stub a generic method, try `onGeneric` instead.", e)
+            throw MockitoKotlinException("NullPointerException thrown when stubbing.\nThis may be due to two reasons:\n\t- The method you're trying to stub threw an NPE: look at the stack trace below;\n\t- You're trying to stub a generic method: try `onGeneric` instead.", e)
         }
     }
 }

mockito-kotlin/src/test/kotlin/test/Classes.kt

@@ -36,6 +36,8 @@ open class Open {
     }
 
     open fun stringResult() = "Default"
+
+    fun throwsNPE(): Any = throw NullPointerException("Test")
 }
 
 class Closed

mockito-kotlin/src/test/kotlin/test/MockitoTest.kt

@@ -957,6 +957,17 @@ class MockitoTest : TestBase() {
         expect(mock.stringResult()).toBe("b")
     }
 
+    @Test
+    fun doReturn_throwsNPE() {
+        expectErrorWithMessage("look at the stack trace below") on {
+
+            /* When */
+            mock<Open> {
+                on { throwsNPE() } doReturn "result"
+            }
+        }
+    }
+
     @Test
     fun doReturn_withGenericIntReturnType_on() {
         /* Expect */