showAllPhraseInterpretations fails with NullPointerException

[WolfgangFahl]

With the grammar from ANTLR Issue 994
antlr/antlr4#994

and testing expr with input
a and b or b and not c

clicking the read ambiguity line and right clicking "show all phrase interpretations"

    at org.antlr.v4.runtime.tree.Trees.findNodeSuchThat(Trees.java:270)
    at org.antlr.intellij.plugin.parsing.ParsingUtils.findOverriddenDecisionRoot(ParsingUtils.java:565)
    at org.antlr.intellij.plugin.preview.ShowAmbigTreesDialog.setTrees(ShowAmbigTreesDialog.java:228)
    at org.antlr.intellij.plugin.preview.ShowAmbigTreesDialog.popupAmbigTreesDialog(ShowAmbigTreesDialog.java:127)
    at org.antlr.intellij.plugin.preview.ShowAmbigTreesDialog$3.run(ShowAmbigTreesDialog.java:84)``` 

[WolfgangFahl]

the Context tree ctx is null because the following code might provide null trees as part of the result list:

public static List<ParserRuleContext> getAllPossibleParseTrees(Grammar g, Parser originalParser, TokenStream tokens, int decision, BitSet alts, int startIndex, int stopIndex, int startRuleIndex) throws RecognitionException {
        ArrayList trees = new ArrayList();
        ParserInterpreter parser = deriveTempParserInterpreter(g, originalParser, tokens);

        for(int alt = alts.nextSetBit(0); alt >= 0; alt = alts.nextSetBit(alt + 1)) {
            parser.reset();
            parser.addDecisionOverride(decision, startIndex, alt);
            ParserRuleContext t = parser.parse(startRuleIndex);
            GrammarInterpreterRuleContext ambigSubTree = (GrammarInterpreterRuleContext)Trees.getRootOfSubtreeEnclosingRegion(t, startIndex, stopIndex);
            if(Trees.isAncestorOf(parser.getOverrideDecisionRoot(), ambigSubTree)) {
                ambigSubTree = (GrammarInterpreterRuleContext)parser.getOverrideDecisionRoot();
            }

            trees.add(ambigSubTree);
        }

        return trees;

there should IMHO be a check that ambigSubTree is not null before adding it ... at https://github.com/antlr/antlr4/blob/master/tool/src/org/antlr/v4/tool/GrammarParserInterpreter.java#L323