Update Model.php

Fix TypeError error in the methods: doFind and doFirst when using casts

system/Model.php

@@ -210,7 +210,7 @@ protected function doFind(bool $singleton, $id = null)
             $row = $builder->get()->getResult($this->tempReturnType);
         }
 
-        if ($useCast) {
+        if ($useCast && $row !== null) {
             $row = $this->convertToReturnType($row, $returnType);
 
             $this->tempReturnType = $returnType;