Address @athei's comments

frame/contracts/src/exec.rs

@@ -156,7 +156,7 @@ pub trait Ext: sealing::Sealed {
 		take_old: bool,
 	) -> Result<WriteOutcome, DispatchError>;
 
-	/// Sets new code hash for existing contract
+	/// Sets new code hash for existing contract.
 	fn set_code_hash(&mut self, hash: CodeHash<Self::T>) -> Result<(), DispatchError>;
 
 	/// Returns a reference to the account id of the caller.
@@ -1021,10 +1021,10 @@ where
 	}
 
 	fn set_code_hash(&mut self, hash: CodeHash<Self::T>) -> Result<(), DispatchError> {
-		let top_frame = &mut self.top_frame_mut();
+		increment_refcount::<Self::T>(hash)?;
+		let top_frame = self.top_frame_mut();
 		let prev_hash = top_frame.contract_info().code_hash.clone();
 		top_frame.contract_info().code_hash = hash;
-		increment_refcount::<Self::T>(hash)?;
 		decrement_refcount::<Self::T>(prev_hash)?;
 		Contracts::<Self::T>::deposit_event(Event::ContractCodeUpdated {
 			contract: top_frame.account_id.clone(),

frame/contracts/src/lib.rs

@@ -560,11 +560,11 @@ pub mod pallet {
 		/// A code with the specified hash was removed.
 		CodeRemoved { code_hash: T::Hash },
 
-		/// Contract code has been updated to one specified with code_hash
+		/// A contract code has changed its code.
 		ContractCodeUpdated {
 			/// The contract that has been updated.
 			contract: T::AccountId,
-			/// New code hash that was set for the contract
+			/// New code hash that was set for the contract.
 			code_hash: T::Hash,
 		},
 	}

frame/contracts/src/wasm/code_cache.rs

@@ -122,7 +122,7 @@ pub fn decrement_refcount<T: Config>(code_hash: CodeHash<T>) -> Result<(), Dispa
 ///
 /// # Errors
 ///
-/// DispatchError::CannotLookup is returned if there is no info for specified code_hash
+/// `Error::CodeNotFound` is returned if the specified `code_hash` does not exist.
 pub fn increment_refcount<T: Config>(code_hash: CodeHash<T>) -> Result<(), DispatchError> {
 	<OwnerInfoOf<T>>::mutate(code_hash, |existing| -> Result<(), DispatchError> {
 		if let Some(info) = existing {