ChartShape: Fix crash due to accessing stale pointers

Summary:
Some data pointers where not reset when shape selection changed,
thus accessing data from privously selected shape.

Test:
Add a chart to eg sheets
Undo
Add another shape to chart

Result before: crash
After: No crash

This may have bearing on:
CCBUG: 365238
but needs more testing.

Reviewers: mecir, boemann

Reviewed By: boemann

Tags: KEXI, #calligra:_3.0

Differential Revision: https://phabricator.kde.org/D3455