The answer is very simple: a ComboBox, when it is created (instantiated), has its 'SelectedItem Property set to Null. You can verify this by putting this in your code, and setting a break-point, and examining the value in the variable after this line of code executes:
object defaultComboBoxSelectedItem = comboBox1.SelectedItem;
So, you have some choices to make: you can set the 'SelectedItem Property of the ComboBox when the Form is created, and before
you try and access it, or, you can change the place
in your code in the other Form where the value of ComboBox 'SelectedItem is accessed.
With the first choice, you, in essence, supply a default selection
in the ComboBox: that may be very appropriate, or not appropriate: depends on the context.
Or, you can test for SelectedItem == null, and only return some string value that will indicate its null, if the value is null.
The choice you make will depend on your Application, and the intended interaction between your two Forms you wish to present.