Fix various phpstan errors.

app/Support/Http/Api/ExchangeRateConverter.php

@@ -123,7 +123,7 @@ class ExchangeRateConverter
         }
         app('log')->debug(sprintf('Going to get rate #%d->#%d (%s) from DB.', $from, $to, $date));
 
-        /** @var CurrencyExchangeRate $result */
+        /** @var CurrencyExchangeRate|null $result */
         $result = auth()->user()
                         ->currencyExchangeRates()
                         ->where('from_currency_id', $from)

app/Support/Request/ChecksLogin.php

@@ -78,7 +78,7 @@ trait ChecksLogin
         /** @var User $user */
         $user = auth()->user();
         app('log')->debug('Now in getUserGroup()');
-        /** @var UserGroup $userGroup */
+        /** @var UserGroup|null $userGroup */
         $userGroup = $this->route()->parameter('userGroup');
         if (null === $userGroup) {
             app('log')->debug('Request class has no userGroup parameter, but perhaps there is a parameter.');