fix: ensure _provider exist while using can_transact

Author: 1yam

src/aleph/sdk/chains/ethereum.py

@@ -105,6 +105,8 @@ def switch_chain(self, chain: Optional[Chain] = None):
     def can_transact(self, tx: TxParams, block=True) -> bool:
         balance = self.get_eth_balance()
         try:
+            assert self._provider is not None
+
             estimated_gas = self._provider.eth.estimate_gas(tx)
         except Exception:
             estimated_gas = (