"Invalid call" and "Invalid get index" are the most common runtime errors a beginner will encounter. They are actually the same error that has to do with accessing properties or methods using the dot syntax. You normally do it like this:
object.property # get object's property
object.method() # call object's method
All is well if the variable object does indeed contain a reference to an object posessing a given property or a method. However if the object doesn't have it, one of the above errors will be thrown; invalid get index when accessing a property or invalid call when calling a method.
The worst case of this is when the object is not defined at all, like when you declare a variable but don't initialize it.
So here are some examples:
var s = "hello"
s.my_property
^ Invalid get index "my_property' (on base: 'String') - you're trying to access a non exsistent property of class String named my_property. Built in class String does not have my_property.
var s
s.my_property
^ Invalid get index "my_property' (on base: 'Nil') - you're trying to access a property of a undefined variable, whose value is null or Nil as the error calls it.
var s = null
s.my_property
^ Invalid get index "my_property' (on base: 'Nil') - exactly the same as previous error but here the null value is explicitly assigned
The invalid call error is analogous to this but happens when you call a nonexistent method or the base object on which you're calling it is null.
var s = "hello"
s.my_method()
^ Invalid call. Nonexistent function "my_method' (on base: 'String') - you're trying to call a non exsistent method of class String named my_method. Built in class String does not have my_method.
var s
s.my_method()
^ Invalid call. Nonexistent function "my_method' (on base: 'Nil') - you're trying to call a method on a undefined variable whose value is null.
So when you get one of those errors always check if the variable you're using with the dot syntax is of expected type and its value is not null. The error message is actually quite informative and straightforward, if you know how to read it.
