fix: dde-file-mananger crash, switch show mode

[liyigang1]

dde-file-mananger crash, switch show mode

Log: dde-file-mananger crash, switch show mode

[deepin-ci-robot]

deepin pr auto review

关键摘要:

• 在FileView::setViewMode函数中，d->delegates[static_cast<int>(Global::ViewMode::kListMode)]->setPaintProxy(new ListItemPaintProxy(this));被重复调用，应该移除重复代码。
• setIconSize函数被调用两次，应该移除不必要的调用。
• viewport()->setContentsMargins(0,0,0,0);在setViewMode函数中被调用，但在kExtendMode和kTreeMode的情况下没有设置任何代理，可能会导致未定义的行为。
• setListViewMode函数在kAllViewMode情况下被调用，但在代码更改前已经调用了setListViewMode，这可能是不必要的。
• ListItemDelegate::getRectOfItem函数中，对d->paintProxy的存在性进行了检查，但是没有处理d->paintProxy为nullptr的情况，可能会导致空指针解引用。

是否建议立即修改:
是

[deepin-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: Johnson-zs, liyigang1

The full list of commands accepted by this bot can be found here.

Needs approval from an approver in each of these files:

• debian/deepin/OWNERS

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment