<Swing Dev> [12] RFR JDK-8213843: Changing L&F from Nimbus to Window L&F causes NPE in SwingSet2

[Prasanta Sadhukhan]

Hi All,

Please review a fix for an issue seen where when we switch look and feel 
from Nimbus to
Windows or any other l&f, it causes a NPE in SwingSet2 demo.
NPE is caused by SynthButtonUI.getMinimumSize() calling 
SynthContext.getStyle().getGraphicsUtils()...when getStyle() return null.
This is because when we switch L&F, uninstallDefaults() is called where 
style is set to null.

This is a regression introduced in jdk12 by the fix of
8192888: AllSwingComponentsBaselineTest fails with NullPointerException 
for NimbusLookAndFeel
where caching of SynthButtonUI is done. It seems caching of button ui 
object causes SynthButtoUI.getMinimumSize() to be called again
whereas without caching, the SynthButtonUI class is not invoked when 
Container.layout() lay outs the components when we switch L&F from 
nimbus to something else.

As a fix, I have removed the caching from SynthButtonUI#createUI and 
updating the "style" in getBaseline() if the style is null to address 
above 8192688 issue.
This fixes this issue, JDK-8192888 and
  JDK-8213121: 
<https://bugs.openjdk.java.net/browse/JDK-8213121>javax/swing/GraphicsConfigNotifier/StalePreferredSize.java 
fails on mac10.13

Bug: https://bugs.openjdk.java.net/browse/JDK-8213843
webrev: http://cr.openjdk.java.net/~psadhukhan/8213843/webrev.0/

Regards
Prasanta
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