problem: nullpointer stacktrace if json is just "null"

src/main/kotlin/io/emeraldpay/dshackle/upstream/ethereum/CacheRequested.kt

@@ -46,7 +46,8 @@ class CacheRequested(
         if (params.size != 1) {
             return
         }
-        val parsed = Global.objectMapper.readValue(json, TransactionReceiptJson::class.java)
+        // note: json could be a `null` value
+        val parsed = Global.objectMapper.readValue(json, TransactionReceiptJson::class.java) ?: return
         val value = DefaultContainer<TransactionReceiptJson>(
                 TxId.from(parsed.transactionHash),
                 BlockId.from(parsed.blockHash),

src/test/groovy/io/emeraldpay/dshackle/upstream/ethereum/CacheRequestedSpec.groovy

@@ -42,4 +42,17 @@ class CacheRequestedSpec extends Specification {
         })
     }
 
+    def "Ignore null"() {
+        setup:
+        def caches = Mock(Caches)
+        CacheRequested instance = new CacheRequested(caches)
+        def json = 'null'.bytes
+
+        when:
+        instance.cacheTxReceipt(["0x5929b36be4586c57bd87dfb7ea6be3b985c1f527fa3d69d221604b424aeb4197"], json)
+
+        then:
+        0 * caches.cacheReceipt(_, _)
+    }
+
 }