fixed NPE in FlatUIUtils.isCellEditor() (issue #601)

CHANGELOG.md

@@ -23,6 +23,7 @@ FlatLaf Change Log
   feedback only for left mouse button.
 - ScaledImageIcon: Do not throw exceptions if image was has invalid size (e.g.
   not found). Instead, paint a red rectangle (similar to `FlatSVGIcon`).
+- Fixed NPE in `FlatUIUtils.isCellEditor()`. (issue #601)
 
 
 ## 2.5

flatlaf-core/src/main/java/com/formdev/flatlaf/ui/FlatUIUtils.java

@@ -210,6 +210,9 @@ public class FlatUIUtils
 	}
 
 	public static boolean isCellEditor( Component c ) {
+		if( c == null )
+			return false;
+
 		// check whether used in cell editor (check 3 levels up)
 		Component c2 = c;
 		for( int i = 0; i <= 2 && c2 != null; i++ ) {