fix for '0' gasPrice from web3

if i had some accounts already saved and initSendForm requested "gasPrice" from web3 which didn't exists in this moment yet, returned value was '0'.

This commit fixes that by returning defaultGasPrice instead of '0' and also initializes web3 during Backend.subscribe (before account is shown)

src/actions/BlockchainActions.js

@@ -57,7 +57,7 @@ export const getTokenBalance = (token: Token): PromiseAction<string> => async (d
 export const getGasPrice = (network: string, defaultGasPrice: number): PromiseAction<BigNumber> => async (dispatch: Dispatch): Promise<BigNumber> => {
     try {
         const gasPrice = await dispatch(Web3Actions.getCurrentGasPrice(network));
-        return new BigNumber(gasPrice);
+        return gasPrice === '0' ? new BigNumber(defaultGasPrice) : new BigNumber(gasPrice);
     } catch (error) {
         return new BigNumber(defaultGasPrice);
     }
@@ -175,6 +175,8 @@ export const subscribe = (network: string): PromiseAction<void> => async (dispat
         accounts: [],
         coin: network,
     });
+    // init web3 instance if not exists
+    await dispatch(Web3Actions.initWeb3(network));
 };
 
 // Conditionally subscribe to blockchain backend