Fix SpotBugs NP_ALWAYS_NULL warnings (#4364)

* Fix SpotBugs NP_ALWAYS_NULL warnings in Form and UIFragment

Fixes "Null pointer dereference" warnings reported by SpotBugs.
- In `Form.java`, fix potential NPE in `getFormLayeredPane` and safely handle `formLayeredPane` using a local variable.
- In `UIFragment.java`, initialize `el` before dereferencing it in `buildXMLFromJSONNotation` when the first element is not a String.
- Update `.github/scripts/generate-quality-report.py` to fail on `NP_ALWAYS_NULL` violations.

* Fix SpotBugs NP_ALWAYS_NULL warnings in Form and UIFragment

Fixes "Null pointer dereference" warnings reported by SpotBugs.
- In `Form.java`, fix potential NPE in `getFormLayeredPane` by handling the null case for class `c` correctly and safely handling `formLayeredPane` using a local variable.
- In `UIFragment.java`, initialize `el` before dereferencing it in `buildXMLFromJSONNotation` when the first element is not a String.
- Update `.github/scripts/generate-quality-report.py` to fail on `NP_ALWAYS_NULL` violations.

---------

Co-authored-by: google-labs-jules[bot] <161369871+google-labs-jules[bot]@users.noreply.github.com>

Author: google-labs-jules[bot]

.github/scripts/generate-quality-report.py

@@ -758,6 +758,7 @@ def main() -> None:
     spotbugs, _, _ = parse_spotbugs()
     if spotbugs:
         forbidden_rules = {
+            "NP_ALWAYS_NULL",
             "RCN_REDUNDANT_NULLCHECK_OF_NONNULL_VALUE",
             "RCN_REDUNDANT_NULLCHECK_OF_NULL_VALUE",
             "UWF_FIELD_NOT_INITIALIZED_IN_CONSTRUCTOR",

CodenameOne/src/com/codename1/ui/Form.java

@@ -1568,12 +1568,13 @@ public void paintBackgrounds(Graphics g) {
             formLayeredPane.setHeight(getHeight());
             formLayeredPane.setShouldLayout(false);
         }
+        Container flp = formLayeredPane;
         if (c == null) {
             // NOTE: We need to use getChildrenAsList(true) rather than simply iterating
             // over layeredPaneImpl because the latter won't find components while an animation
             // is in progress.... We could end up adding a whole bunch of layered panes
             // by accident
-            for (Component cmp : formLayeredPane.getChildrenAsList(true)) {
+            for (Component cmp : flp.getChildrenAsList(true)) {
                 if (cmp.getClientProperty("cn1$_cls") == null) {
                     return (Container) cmp;
                 }
@@ -1583,16 +1584,16 @@ public void paintBackgrounds(Graphics g) {
             cnt.setWidth(getWidth());
             cnt.setHeight(getHeight());
             cnt.setShouldLayout(false);
-            cnt.setName("FormLayer: " + c.getName());
-            formLayeredPane.add(cnt);
+            cnt.setName("FormLayer: null");
+            flp.add(cnt);
             return cnt;
         }
         String n = c.getName();
         // NOTE: We need to use getChildrenAsList(true) rather than simply iterating
         // over layeredPaneImpl because the latter won't find components while an animation
         // is in progress.... We could end up adding a whole bunch of layered panes
         // by accident
-        for (Component cmp : formLayeredPane.getChildrenAsList(true)) {
+        for (Component cmp : flp.getChildrenAsList(true)) {
             if (n.equals(cmp.getClientProperty("cn1$_cls"))) {
                 return (Container) cmp;
             }
@@ -1603,9 +1604,9 @@ public void paintBackgrounds(Graphics g) {
         cnt.setShouldLayout(false);
         cnt.setName("FormLayer: " + c.getName());
         if (top) {
-            formLayeredPane.add(cnt);
+            flp.add(cnt);
         } else {
-            formLayeredPane.addComponent(0, cnt);
+            flp.addComponent(0, cnt);
         }
         cnt.putClientProperty("cn1$_cls", n);
         return cnt;

CodenameOne/src/com/codename1/ui/UIFragment.java

@@ -912,6 +912,7 @@ private static Element buildXMLFromJSONNotation(Object o) throws IOException {
                             el.addChild(buildXMLFromJSONNotation(first));
                         }
                     } else {
+                        el = new Element("flow");
                         el.addChild(buildXMLFromJSONNotation(first));
                     }
                     for (int i = 1; i < len; i++) {