ETCM-636: Fix null pointer on getBestBlock

Author: AnastasiiaL

src/main/scala/io/iohk/ethereum/blockchain/sync/fast/FastSync.scala

@@ -823,8 +823,8 @@ class FastSync(
         val bestReceivedBlock = fullBlocks.maxBy(_.number)
         val lastStoredBestBlockNumber = appStateStorage.getBestBlockNumber()
         if (lastStoredBestBlockNumber < bestReceivedBlock.number) {
-          blockchain.saveBestKnownBlocks(bestReceivedBlock.number)
           appStateStorage.putBestBlockNumber(bestReceivedBlock.number).commit()
+          blockchain.saveBestKnownBlocks(bestReceivedBlock.number)
         }
         syncState = syncState.copy(lastFullBlockNumber = bestReceivedBlock.number.max(lastStoredBestBlockNumber))
       }

src/test/scala/io/iohk/ethereum/db/dataSource/DataSourceTestBehavior.scala

@@ -2,9 +2,9 @@ package io.iohk.ethereum.db.dataSource
 
 import java.io.File
 import java.nio.file.Files
-
 import io.iohk.ethereum.ObjectGenerators
 import io.iohk.ethereum.db.dataSource.DataSource.{Key, Namespace, Value}
+import io.iohk.ethereum.db.dataSource.RocksDbDataSource.RocksDbDataSourceClosedException
 import org.scalatest.flatspec.AnyFlatSpec
 import org.scalatestplus.scalacheck.ScalaCheckPropertyChecks
 
@@ -48,6 +48,15 @@ trait DataSourceTestBehavior extends ScalaCheckPropertyChecks with ObjectGenerat
       }
     }
 
+    it should "throw an exception if the rocksdb storage is unavailable" in {
+      withDir { path =>
+        val dataSource = createDataSource(path)
+        val someByteString = byteStringOfLengthNGen(KeySizeWithoutPrefix).sample.get
+        dataSource.destroy()
+        assertThrows[RocksDbDataSourceClosedException](dataSource.update(prepareUpdate(toUpsert = Seq(someByteString -> someByteString))))
+      }
+    }
+
     it should "allow to remove keys" in {
       val key1 = byteStringOfLengthNGen(KeySizeWithoutPrefix).sample.get
       val key2 = byteStringOfLengthNGen(KeySizeWithoutPrefix).sample.get
@@ -111,5 +120,4 @@ trait DataSourceTestBehavior extends ScalaCheckPropertyChecks with ObjectGenerat
     }
   }
   // scalastyle:on
-
 }