package org.antlr.intellij.plugin.structview;

import com.intellij.ide.structureView.StructureViewBuilder;
import com.intellij.ide.structureView.StructureViewModel;
import com.intellij.ide.structureView.StructureViewModelBase;
import com.intellij.ide.structureView.StructureViewTreeElement;
import com.intellij.ide.structureView.TreeBasedStructureViewBuilder;
import com.intellij.ide.structureView.impl.common.PsiTreeElementBase;
import com.intellij.lang.PsiStructureViewFactory;
import com.intellij.openapi.editor.Editor;
import com.intellij.openapi.vfs.VirtualFile;
import com.intellij.psi.PsiElement;
import com.intellij.psi.PsiFile;
import org.antlr.intellij.plugin.ANTLRv4FileRoot;
import org.jetbrains.annotations.NotNull;
import org.jetbrains.annotations.Nullable;

import java.util.Collection;
import java.util.Collections;

public class ANTLRv4StructureViewFactory implements PsiStructureViewFactory {
	/** fake a blank Treeview with a warning */
	public static class DummyViewTreeElement extends PsiTreeElementBase<PsiElement> {
		public DummyViewTreeElement(PsiElement psiElement) {
			super(psiElement);
		}

		@NotNull
		@Override
		public Collection<StructureViewTreeElement> getChildrenBase() {
			return Collections.emptyList();
		}

		@Nullable
		@Override
		public String getPresentableText() {
			return "Sorry .g not supported (use .g4)";
		}
	}

	@Nullable
    @Override
    public StructureViewBuilder getStructureViewBuilder(final PsiFile psiFile) {
        return new TreeBasedStructureViewBuilder() {
            @NotNull
            @Override
            public StructureViewModel createStructureViewModel(@NotNull Editor editor) { // the nondeprecated method in 13 won't compile in 12.
				VirtualFile grammarFile = psiFile.getVirtualFile();
				if ( grammarFile==null || !grammarFile.getName().endsWith(".g4") ) {
					return new StructureViewModelBase(psiFile, new DummyViewTreeElement(psiFile));
				}
                return new ANTLRv4StructureViewModel((ANTLRv4FileRoot)psiFile);
            }
        };
    }
}

package org.antlr.intellij.plugin.structview;

import com.intellij.ide.structureView.StructureViewBuilder;
import com.intellij.ide.structureView.StructureViewModel;
import com.intellij.ide.structureView.StructureViewModelBase;
import com.intellij.ide.structureView.StructureViewTreeElement;
import com.intellij.ide.structureView.TreeBasedStructureViewBuilder;
import com.intellij.ide.structureView.impl.common.PsiTreeElementBase;
import com.intellij.lang.PsiStructureViewFactory;
import com.intellij.openapi.editor.Editor;
import com.intellij.openapi.vfs.VirtualFile;
import com.intellij.psi.PsiElement;
import com.intellij.psi.PsiFile;
import org.antlr.intellij.plugin.ANTLRv4FileRoot;
import org.jetbrains.annotations.NotNull;
import org.jetbrains.annotations.Nullable;

import java.util.Collection;
import java.util.Collections;

public class ANTLRv4StructureViewFactory implements PsiStructureViewFactory {
	/** fake a blank Treeview with a warning */
	public static class DummyViewTreeElement extends PsiTreeElementBase<PsiElement> {
		public DummyViewTreeElement(PsiElement psiElement) {
			super(psiElement);
		}

		@NotNull
		@Override
		public Collection<StructureViewTreeElement> getChildrenBase() {
			return Collections.emptyList();
		}

		@Nullable
		@Override
		public String getPresentableText() {
			return "Sorry .g not supported (use .g4)";
		}
	}

	@Nullable
    @Override
    public StructureViewBuilder getStructureViewBuilder(final PsiFile psiFile) {
        return new TreeBasedStructureViewBuilder() {

	        @NotNull
            @Override
            public StructureViewModel createStructureViewModel(@Nullable Editor editor) { // the nondeprecated method in 13 won't compile in 12.
				VirtualFile grammarFile = psiFile.getVirtualFile();
				if ( grammarFile==null || !grammarFile.getName().endsWith(".g4") ) {
					return new StructureViewModelBase(psiFile, new DummyViewTreeElement(psiFile));
				}
                return new ANTLRv4StructureViewModel((ANTLRv4FileRoot)psiFile);
            }

        };
    }
}

Fix to #177 Issue : Problem loading Structure View in Intellij 14.1 fixed : @NotNull to @Nullable, my fault