Just having the fields on a form should not cause any errors like you are describing.
The variable "f" is expected to be field object that has used the "this.getField()" method to obtain the field object for the named field but the field does not or no longer exist.
Have you renamed any fields. if so check your calculations to make sure they are still using the correct field.
TypeError: f is null
TypeError: f is null - the error message
1051:byteCodeTool - the line number and the possible source of the code.
Your error is coming from the precompiled code provided by Adobe as part of Acrobat Reader. The code is several thousand lines long and provides all the predefined formats and calculations.
Another common error deals with the result of a calculation not matching the format for the field. For this error I change the format to "None" to see what the result value is. Then it is a matter of trying to figure out why it is displaying what is being displayed. Division by zero is NaN and mixing numbers and strings creates a long number or a string of numbers and symbols.