[TYPO3-mvc] Create an object in multi step form

[Mam Violeng]

Hi All,

I have a model of object with many fields and want to write the actions 
to create this object in multi step form (about 5 step). example:

step 1 have 1 field "field1"
Step 2 have 1 field "field2"
..
Step 5 have 1 field "field5"

I have separate 5 difference action like "step1Action" to "step5Action"

here is my sample code in each action in the controller

function step1Action (Tx_ExtKey_Domain_Model_Project $project){
         $this->view->assign('project',$project);
}
function step2Action (Tx_ExtKey_Domain_Model_Project $project){
         $this->view->assign('project',$project);
}
..

here is my fluid form (step1.html)
<f:form method="post" controller="Project" action="step2" name="project" 
object="{project}">
     <label for="field1">Field1</label><br />
     <f:form.textbox property="field1" />
     <f:form.submit class="submit" value="Submit"/>
</f:form>
..
and other step almost the same template only different action.

Here is my problem now:
When I submit from step1 to step2 than it work properly but when I 
submit from step2 to step3 then appear this error message.
*****
The value must be of type "Tx_ExtKey_Domain_Model_Project", but was of 
type "array"
*****

I don't know if I go on the right way.
Could any one please give me some hint how to implement this?

Thanks,
Violeng Mam