ProjectExplorer: Avoid nullptr access

The path chooser for the compiler command is
initialized only for manually defined compilers.
Skip the call to apply() on the options page
when processing auto-detected toolchains to
avoid a crash.

Change-Id: Ib795d233f0372ef74e42b323544b0bbd95c27646
Reviewed-by: Christian Kandeler <christian.kandeler@qt.io>

src/plugins/projectexplorer/toolchainoptionspage.cpp

@@ -463,7 +463,7 @@ void ToolChainOptionsWidget::apply()
             for (TreeItem *item : *parent) {
                 auto tcItem = static_cast<ToolChainTreeItem *>(item);
                 Q_ASSERT(tcItem->toolChain);
-                if (tcItem->widget)
+                if (!tcItem->toolChain->isAutoDetected() && tcItem->widget)
                     tcItem->widget->apply();
                 tcItem->changed = false;
                 tcItem->update();