Backend: handle other Eth server null responses

Raise WeirdNullResponseException on all possible null responses
or responses that contain null value from Ethereum servers.

src/GWallet.Backend/Ether/EtherServer.fs

@@ -482,7 +482,10 @@ module Server =
                             let! cancelToken = Async.CancellationToken
                             let task =
                                 web3.Eth.Transactions.GetTransactionCount.SendRequestAsync(address, null, cancelToken)
-                            return! Async.AwaitTask task
+                            let! txCount = Async.AwaitTask task
+                            if isNull txCount then
+                                raise <| AbnormalNullValueInJsonResponseException "Weird null response from tx count job"
+                            return txCount
                         }
                 GetRandomizedFuncs currency web3Func
             return! faultTolerantEtherClient.Query
@@ -655,7 +658,10 @@ module Server =
                             let! cancelToken = Async.CancellationToken
                             let task =
                                 contractHandler.EstimateGasAsync<TransferFunction>(transferFunctionMsg, cancelToken)
-                            return! Async.AwaitTask task
+                            let! fee = Async.AwaitTask task
+                            if isNull fee then
+                                raise <| AbnormalNullValueInJsonResponseException "Weird null response from transfer fee job"
+                            return fee
                     }
                 GetRandomizedFuncs account.Currency web3Func
             return! faultTolerantEtherClient.Query
@@ -678,6 +684,8 @@ module Server =
                             let! cancelToken = Async.CancellationToken
                             let task = web3.Eth.GasPrice.SendRequestAsync(null, cancelToken)
                             let! hexBigInteger = Async.AwaitTask task
+                            if isNull hexBigInteger then
+                                raise <| AbnormalNullValueInJsonResponseException "Weird null response from gas price job"
                             if hexBigInteger.Value = BigInteger 0 then
                                 return failwith "Some server returned zero for gas price, which is invalid"
                             return hexBigInteger
@@ -708,7 +716,12 @@ module Server =
                             let! cancelToken = Async.CancellationToken
                             let task =
                                 web3.Eth.Transactions.SendRawTransaction.SendRequestAsync(transaction, null, cancelToken)
-                            return! Async.AwaitTask task
+                            let! response = Async.AwaitTask task
+                            if isNull response then
+                                raise <|
+                                    AbnormalNullValueInJsonResponseException
+                                        "Weird null response from broadcast transaction job"
+                            return response
                         }
                 GetRandomizedFuncs currency web3Func
             try
@@ -739,6 +752,10 @@ module Server =
                         let task =
                             web3.TransactionManager.TransactionReceiptService.PollForReceiptAsync(txHash, cancelToken)
                         let! transactionReceipt = Async.AwaitTask task
+                        if isNull transactionReceipt || isNull transactionReceipt.GasUsed || isNull transactionReceipt.Status then
+                            raise <| 
+                                AbnormalNullValueInJsonResponseException
+                                    (SPrintF1 "Weird null response when getting details from tx receipt (%A)" transactionReceipt)
                         return {
                             GasUsed = transactionReceipt.GasUsed.Value
                             Status = transactionReceipt.Status.Value