issue audacity#5357: Change of format of a duration in a generator ca…

…n cause crash

With a screen reader running, in one of the the built in generators, for example chirp, changing the format of the duration can crash Audacity.

Problem:
NumericTextCtrlAx::GetName(), uses NumericTextCtrl::GetFocusedField() to get the focused field. It then uses that as in index for a vector.
However, NumericTextCtrl::UpdateAutoFocus() does not update the value of mLastField, which is returned by NumericTextCtrl::GetFocusedField().
So after a change of format, GetFocusedField() can return a value which does not exist in the new format.
This can cause Audacity to crash. This happens from Audacity 3.3.0, and I don't know why it didn't in versions before this.

Possible fixes:
1. Change NumericTextCtrl::UpdateAutoFocus() so that it updates mLastField.
2. Given that NumericTextCtrl::GetFocusedField() is only used by the accessibility object, and that the focused field can be derived for NumericTextCtrl::GetFocusedDigit(), remove NumericTextCtrl::GetFocusedField() and NumericTextCtrl::mLastField.

The second fix was implemented, as it will reduce the chance of errors in this area in the future.

src/widgets/NumericTextCtrl.cpp

@@ -156,7 +156,6 @@ NumericTextCtrl::NumericTextCtrl(
    mBackgroundBitmap{},
    mDigitFont{},
    mLabelFont{},
-   mLastField(1),
    mAutoPos(options.autoPos)
    , mType(type)
 {
@@ -913,7 +912,6 @@ void NumericTextCtrl::SetFieldFocus(int  digit)
       return;
    }
    mFocusedDigit = digit;
-   mLastField = mFormatter->GetDigitInfos()[mFocusedDigit].field + 1;
 
    GetAccessible()->NotifyEvent(wxACC_EVENT_OBJECT_FOCUS,
                                 this,
@@ -1138,7 +1136,7 @@ wxAccStatus NumericTextCtrlAx::GetName(int childId, wxString *name)
    auto & mFieldValueStrings = mCtrl->mFieldValueStrings;
 
    wxString ctrlString = mCtrl->GetString();
-   int field = mCtrl->GetFocusedField();
+   int field = mDigits[mCtrl->GetFocusedDigit()].field + 1;
 
    // Return the entire string including the control label
    // when the requested child ID is wxACC_SELF.  (Mainly when

src/widgets/NumericTextCtrl.h

@@ -108,7 +108,6 @@ class AUDACITY_DLL_API NumericTextCtrl final
    // this control returns to the program, so you can specify.
    void SetInvalidValue(double invalidValue);
 
-   int GetFocusedField() { return mLastField; }
    int GetFocusedDigit() { return mFocusedDigit; }
 
 private:
@@ -166,8 +165,6 @@ class AUDACITY_DLL_API NumericTextCtrl final
    int            mHeight;
    int            mButtonWidth;
 
-   int            mLastField;
-
    int            mFocusedDigit { 0 };
 
    // If true, the focus will be set to the first non-zero digit