Error Evaluating UI Expression Expression evaluation error in rule 'vfm_addvehicleform' at function a!dropdownField [line 18]: A null parameter has been passed.

I'm running through Instructions as described in the course. I have created the site and when launching the site I get this error "Error Evaluating UI Expression Expression evaluation error in rule 'vfm_addvehicleform' at function a!dropdownField [line 18]: A null parameter has been passed." but I haven't created an expression rule 'vfm_addvehicleform', thus where should i look for this?. 'a!dropdownField [line 18]' I have also checked this in the contents interface and the line 18 is a boolean value. Thus I request to provide help to solve this. 

The below is the code for form interface

=a!formLayout(
label: "Add Vehicle to Fleet",
instructions: "",
contents: {
a!columnsLayout(
columns: {
a!columnLayout(
contents: {
a!dropdownField(
label: "Category",
labelPosition: "ABOVE",
placeholderLabel: "--- Select a category ---",
choiceLabels: {"Small", "Economy", "Large", "Luxury"},
choiceValues: {"small", "Economy", "Large", "Luxury"},
value: ri!category,
saveinto: ri!category,
showwhen: true,
required: true,
validations: {}
),
a!textField(
label: "Make",
labelPosition: "ABOVE",
placeholder: "Make of the Vehicle",
value: ri!Make,
saveInto: ri!Make,
refreshAfter: "UNFOCUS",
required: true,
validations: {}
),
a!textField(
label: "Model ",
labelPosition: "ABOVE",
placeholder: "Model of the Vehicle",
value: ri!Model,
saveInto: ri!Model,
refreshAfter: "UNFOCUS",
required: true,
validations: {
null
}
),
a!textField(
label: "License Plate",
labelPosition: "ABOVE",
placeholder: "License plate no.",
value: ri!VehicleLicensePlate,
saveInto: ri!VehicleLicensePlate,
refreshAfter: "UNFOCUS",
required: ri!VehicleLicensePlate=7,
validations: {}
),
a!fileUploadField(
label: "Picture",
labelPosition: "ABOVE",
target: cons!VFM_VEHICLES_IMAGES_FOLDER,
value: ri!Image,
saveInto: ri!Image,
validations: {}
)
}
),
a!columnLayout(
contents: {
a!integerField(
label: "Mileage",
labelPosition: "ABOVE",
placeholder: "Mileage of a Vehicle",
value: ri!Mileage,
saveInto: ri!Mileage,
refreshAfter: "UNFOCUS",
required: ri!Mileage> 100000,
validations: {}
),
a!paragraphField(
label: "Condition",
labelPosition: "ABOVE",
placeholder: "Condition of the Vehicle",
value: ri!VehicleCondition,
saveInto: ri!VehicleCondition,
refreshAfter: "UNFOCUS",
required: true,
height: "MEDIUM",
validations: {}
)
}
)
}
)
},
buttons: a!buttonLayout(
primaryButtons: {
a!buttonWidgetSubmit(
label: "Submit",
saveInto: {},
style: "PRIMARY"
)
},
secondaryButtons: {
a!buttonWidgetSubmit(
label: "Cancel",
value: true,
saveInto: ri!cancel,
style: "NORMAL",
skipValidation: true
)
}
),
validations: {}
)