Text Line Details — Clicking in "Begin text"

• Add a line object to a score or select a line object in a score
• In the inspector check "Begin text"
• Click in the Text Field

Result: The "Begin text" box becomes unchecked and and accordingly the Text field goes out of view.

• Reselect the line object
• Check "Begin text"
• Click in the Text Field

This time you can enter text.