Simplify access to exception data & null safety fixes

Follow up to !5 (merged) & !6 (merged).

With the recent null safety changes in place, it was found that the reason() method was being prevented from being accessible (not listed as a usable method, treated as if it does not exist [null checks did not help]). It is possible that it is happening as all possible exceptions would also need that method by name with a return data type of String. But that is just speculation of what might be going on.

To get around this in a way that is not so complicated, I instead made it private and passed it onto toString(). This will mean that the custom exception name will not show up in logs and exception data may get altered rather than showing in full when using debug mode (running with debugging).

In addition:

• Fix was made to filesMap regarding null safety
• Published disk_space_ns (null safety) and replaced disk_space with that in pubspec

NULL SAFETY NOTE:

• file_chooser package needs to either be migrated or replaced by file_selector