Error displaying Interface

Hi,

Working on the Advance section of the Appian Developer course and the Exercise is to build an interface using expressions to add new items to the Office Supply Inventory.

I am seeing the following error and it is not displaying the interface in the preview section.  Kindly pls help!!

Could not display interface. Please check definition and inputs. Interface Definition: Expression evaluation error at function a!columnLayout [line 4]: The contents field on a column layout cannot contain a column layout. Received a column layout at index 1.

The code I am using is:

a!formLayout(
label: "Add Item to Inventory",
contents: {
a!columnLayout(
contents: {
a!columnLayout(
contents: {
a!sectionLayout(
label: "Item Information",
contents: {
a!textField(
label: "Item Name",
value: ri!item.name,
saveInto: ri!item.name,
required: true
),
a!paragraphField(
label: "Description",
value: ri!item.description,
saveInto: ri!item.description,
required: true
),
a!floatingPointField(
label: "Cost",
value: ri!item.cost,
saveInto: ri!item.cost,
required: true
),
a!dropdownField(
label: "Color",
labelPosition: "ABOVE",
placeholder: "Select color from the list---",
choiceLabels: cons!ADV_color_list,
choiceValues: cons!ADV_color_list,
value: ri!item.color,
saveInto: ri!item.color,
required:true
)
}
)
}
),
a!columnLayout(contents: {
a!fileUploadField()
}
)
}
)
},
buttons: a!buttonLayout(
primaryButtons:
a!buttonWidget(
label: "Submit",
submit:true(),
style: "PRIMARY"
),
secondaryButtons:
a!buttonWidget(
label: "Cancel",
value: true,
saveInto: ri!cancel,
submit: true(),
style: "NORMAL",
validate: false
)

)
)

Thanks,
Suneetha.