Is there an Editor in Platform Designer?

If you go to the Files tab of the Project Navigator, instead of the Hierarchy tab, you can double-click a source file to open and edit it.

However, for a custom component in Platform Designer, what you're seeing in the Project Navigator is the generated output of Platform Designer since you added this as a custom component to a system. That's why the original source file is not opening from the Project Navigator; Quartus sees the generated output, which is a copy of the original.

The only sure way to open the source for editing is to do File -> Open. You'll still have to regenerate the _hw.tcl file and the system anyway.