Don't crash in wx{File,Dir}PickerCtrl::GetTextCtrlValue()

If there is no text field, assert and return empty string instead of
just crashing.

Closes #14078.

src/common/filepickercmn.cpp

@@ -203,6 +203,8 @@ bool wxFilePickerCtrl::Create(wxWindow *parent,
 
 wxString wxFilePickerCtrl::GetTextCtrlValue() const
 {
+    wxCHECK_MSG( m_text, wxString(), "Can't be used if no text control" );
+
     // filter it through wxFileName to remove any spurious path separator
     return wxFileName(m_text->GetValue()).GetFullPath();
 }
@@ -241,6 +243,8 @@ bool wxDirPickerCtrl::Create(wxWindow *parent,
 
 wxString wxDirPickerCtrl::GetTextCtrlValue() const
 {
+    wxCHECK_MSG( m_text, wxString(), "Can't be used if no text control" );
+
     // filter it through wxFileName to remove any spurious path separator
     return wxFileName::DirName(m_text->GetValue()).GetPath();
 }