MultilineEntry with text wrapping panics when removing letters (cursor wrong position)

[fpabl0]

Describe the bug:

MultilineEntry with text wrapping panics when removing letters, because it suddenly move the cursor to a invalid position.

To Reproduce:

Steps to reproduce the behaviour:

1. Run the example code.
2. Scroll down the Entry until the bottom.
3. Try to remove the last line.
4. Cursor will be moved to an invalid position.
5. Press Backspace key again.
6. Panics.

Screenshots:

multiline_bug.mp4

Example code:

func main() {
	a := app.New()
	w := a.NewWindow("Hello")
	w.Resize(fyne.NewSize(200, 200))

	e := widget.NewMultiLineEntry()
	e.Wrapping = fyne.TextWrapWord
	e.SetText("Hello\ntesting\ntesting\ntesting\ntesting...")
	w.SetContent(container.NewVBox(e))

	w.ShowAndRun()
}

Device (please complete the following information):

• OS: MacOS
• Version: 10.15.4 Catalina
• Go version: 1.16
• Fyne version: develop @ 8a67b5f

[andydotxyz]

Fixed on develop